crucible-llvm-cli: Support for overrides; loading and calling functions

[langston-barrett]

• Exports low-level overrides from crucible-llvm that can be registered without having to construct an L.Declare
• Uses the above to register an override for malloc inside crucible-llvm-syntax
• Removes the L.Type from the LLVM operation for loading a function handle from a pointer
• Adds concrete syntax for the LLVM syntax extension operation of loading a function handle from a pointer (crucible-llvm-syntax: Concrete syntax for remaining LLVM syntax formers #1118)
• Adds concrete syntax for resolving LLVM global names to pointers (crucible-llvm-syntax: Concrete syntax for remaining LLVM syntax formers #1118)
• Adds a simulation test to crucible-llvm-cli that calls the malloc override

[langston-barrett]

It's possible that this will need a tweak here or there to pass CI, but it's generally ready for review.

[RyanGlScott]

I'm broadly in favor of this, although I'm definitely feeling the itch for more documentation.

[langston-barrett]

Would you be OK with turning the llvm-pretty Type field into a Maybe Type and adding accompanying documentation stating that it is solely used for error-message purposes? That way, we can pretty-print it if it's available, but we can otherwise ignore it.

Oh yeah, that's a much better idea :)