Add support for externs, pointers (ffi), casts

[edg-l]

Adds the following:

• Support to declare extern functions, like malloc.
• Support for pointers to use for ffi in the std
• Support for casts (needed to use pointers in a useful way)

Todo:

• [ ] Pointer arithmetic

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.63158% with 252 lines in your changes are missing coverage. Please review.

Project coverage is 61.83%. Comparing base (7ccb2cc) to head (99269b4).

Files	Patch %	Lines
crates/concrete_ir/src/lowering.rs	56.65%	127 Missing :warning:
crates/concrete_codegen_mlir/src/codegen.rs	55.26%	68 Missing :warning:
crates/concrete_ir/src/lib.rs	32.60%	31 Missing :warning:
crates/concrete_ir/src/lowering/prepass.rs	50.00%	15 Missing :warning:
crates/concrete_check/src/lib.rs	0.00%	10 Missing :warning:
crates/concrete_ast/src/types.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## bench #95 +/- ## ========================================== - Coverage 64.29% 61.83% -2.46% ========================================== Files 33 33 Lines 3355 3650 +295 ========================================== + Hits 2157 2257 +100 - Misses 1198 1393 +195 ```

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[github-actions[bot]]

Benchmarking factorial

Running 5000000 iterations Using input value: 20 Concrete Result = 2432902008176640000 Time taken : 59.40 ms Rust Result = 2432902008176640000 Time taken : 60.08 ms

Benchmarking fib

Running 5000 iterations Using input value: 20 Concrete Result = 6765 Time taken : 135.91 ms Rust Result = 6765 Time taken : 85.22 ms

[unbalancedparentheses]

This needs to be reviewed in terms of design. @igaray @juanbono let's jump on a call tomorrow.