Add solver capability for synthesizing splices of ABI compatible packages. (#46729)

This PR provides complementary 2 features:
1. An augmentation to the package language to express ABI compatibility relationships among packages. 
2. An extension to the concretizer that can synthesize splices between ABI compatible packages.

1.  The `can_splice` directive and ABI compatibility 
We augment the package language with a single directive: `can_splice`. Here is an example of a package `Foo` exercising the `can_splice` directive:

class Foo(Package):
    version("1.0")
    version("1.1")
    variant("compat", default=True)
    variant("json", default=False)
    variant("pic", default=False)
    can_splice("foo@1.0", when="@1.1")
    can_splice("bar@1.0", when="@1.0+compat")
    can_splice("baz@1.0+compat", when="@1.0+compat", match_variants="*")
    can_splice("quux@1.0", when=@1.1~compat", match_variants="json")

Explanations of the uses of each directive: 
- `can_splice("foo@1.0", when="@1.1")`:  If `foo@1.0` is the dependency of an already installed spec and `foo@1.1` could be a valid dependency for the parent spec, then `foo@1.1` can be spliced in for `foo@1.0` in the parent spec.
- `can_splice("bar@1.0", when="@1.0+compat")`: If `bar@1.0` is the dependency of an already installed spec and `foo@1.0+compat` could be a valid dependency for the parent spec, then `foo@1.0+compat` can be spliced in for `bar@1.0+compat` in the parent spec
-  `can_splice("baz@1.0", when="@1.0+compat", match_variants="*")`: If `baz@1.0+compat` is the dependency of an already installed spec and `foo@1.0+compat` could be a valid dependency for the parent spec, then `foo@1.0+compat` can be spliced in for `baz@1.0+compat` in the parent spec, provided that they have the same value for all other variants (regardless of what those values are). 
-  `can_splice("quux@1.0", when=@1.1~compat", match_variants="json")`:If `quux@1.0` is the dependency of an already installed spec and `foo@1.1~compat` could be a valid dependency for the parent spec, then `foo@1.0~compat` can be spliced in for `quux@1.0` in the parent spec, provided that they have the same value for their `json` variant. 

2. Augmenting the solver to synthesize splices
### Changes to the hash encoding in `asp.py`
Previously, when including concrete specs in the solve, they would have the following form:

installed_hash("foo", "xxxyyy")
imposed_constraint("xxxyyy", "foo", "attr1", ...)
imposed_constraint("xxxyyy", "foo", "attr2", ...)
% etc. 

Concrete specs now have the following form:
installed_hash("foo", "xxxyyy")
hash_attr("xxxyyy", "foo", "attr1", ...)
hash_attr("xxxyyy", "foo", "attr2", ...)

This transformation allows us to control which constraints are imposed when we select a hash, to facilitate the splicing of dependencies. 

2.1 Compiling `can_splice` directives in `asp.py`
Consider the concrete spec:
foo@2.72%gcc@11.4 arch=linux-ubuntu22.04-icelake build_system=autotools ^bar ...
It will emit the following facts for reuse (below is a subset)

installed_hash("foo", "xxxyyy")
hash_attr("xxxyyy", "hash", "foo", "xxxyyy")
hash_attr("xxxyyy", "version", "foo", "2.72")
hash_attr("xxxyyy", "node_os", "ubuntu22.04")
hash_attr("xxxyyy", "hash", "bar", "zzzqqq")
hash_attr("xxxyyy", "depends_on", "foo", "bar", "link")

Rules that derive abi_splice_conditions_hold will be generated from 
use of the `can_splice` directive. They will have the following form:
can_splice("foo@1.0.0+a", when="@1.0.1+a", match_variants=["b"]) --->

abi_splice_conditions_hold(0, node(SID, "foo"), "foo", BaseHash) :-
  installed_hash("foo", BaseHash),
  attr("node", node(SID, SpliceName)),
  attr("node_version_satisfies", node(SID, "foo"), "1.0.1"),
  hash_attr("hash", "node_version_satisfies", "foo", "1.0.1"),
  attr("variant_value", node(SID, "foo"), "a", "True"),
  hash_attr("hash", "variant_value", "foo", "a", "True"),
  attr("variant_value", node(SID, "foo"), "b", VariVar0),
  hash_attr("hash", "variant_value", "foo", "b", VariVar0).


2.2 Synthesizing splices in `concretize.lp` and `splices.lp`

The ASP solver generates "splice_at_hash" attrs to indicate that a particular node has a splice in one of its immediate dependencies. 

Splices can be introduced in the dependencies of concrete specs when `splices.lp` is conditionally loaded (based on the config option `concretizer:splice:True`. 

2.3 Constructing spliced specs in `asp.py`

The method `SpecBuilder._resolve_splices` implements a top-down memoized implementation of hybrid splicing. This is an optimization over the more general `Spec.splice`, since the solver gives a global view of exactly which specs can be shared, to ensure the minimal number of splicing operations. 

Misc changes to facilitate configuration and benchmarking 
- Added the method `Solver.solve_with_stats` to expose timers from the public interface for easier benchmarking 
- Added the boolean config option `concretizer:splice` to conditionally load splicing behavior 

Co-authored-by: Greg Becker <becker33@llnl.gov>

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