refactor: pair callables with signatures

[baszalmstra]

Why

Indirect calls relied on LLVM function-pointer element types to recover the call signature. Opaque pointers erase that type, so a loaded address is not enough to emit a valid call.

What

• add a Callable newtype that owns a function pointer and its FunctionType
• retain signatures beside dispatch-table entries
• use the same call path for direct functions and runtime-linked functions
• remove unused target and context state from the finalized dispatch table

Testing

• pixi run check with RUSTFLAGS=-Dwarnings
• pixi run cargo test -p mun_codegen --lib
• pixi run cargo test -p mun_codegen --test abi

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.46%. Comparing base (c65a8f3) to head (55f1c13).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           bas/place-projections     #607      +/-   ##
=========================================================
+ Coverage                  86.58%   87.46%   +0.87%     
=========================================================
  Files                        254      254              
  Lines                      26887    26946      +59     
=========================================================
+ Hits                       23281    23567     +286     
+ Misses                      3606     3379     -227     

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