Externalize builtins and fix bugs

[yamaguchi1024]

• Rename builtins to externs
• Externalize externs
• Fix bug 1: extern did not work on read expressions with index accesses
• Fix bug 2: extern fnarg codegen should be comp_e not comp_fnarg
• Fix bug 3: extern did not work on constants
• Change pattern_match and parse_fragment to take local and global variables in scope.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.02894% with 59 lines in your changes are missing coverage. Please review.

Project coverage is 87.00%. Comparing base (5ca4872) to head (4a523c2).

Files	Patch %	Lines
src/exo/libs/externs.py	76.97%	32 Missing :warning:
src/exo/extern.py	70.00%	6 Missing :warning:
src/exo/parse_fragment.py	60.00%	4 Missing :warning:
src/exo/pyparser.py	86.36%	3 Missing :warning:
src/exo/API_cursors.py	75.00%	2 Missing :warning:
src/exo/LoopIR_unification.py	33.33%	2 Missing :warning:
src/exo/stdlib/inspection.py	33.33%	2 Missing :warning:
tests/test_externs.py	94.59%	2 Missing :warning:
src/exo/API.py	80.00%	1 Missing :warning:
src/exo/LoopIR_compiler.py	95.65%	1 Missing :warning:
... and 4 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #637 +/- ## ======================================== Coverage 86.99% 87.00% ======================================== Files 94 96 +2 Lines 22022 22155 +133 ======================================== + Hits 19159 19276 +117 - Misses 2863 2879 +16 ```

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[SamirDroubi]

I think a more ergonomic way of implementing this feature would be something like the following:

@extern("{a_data} < {b_data} ? {c_data} : {d_data}")
def select(a: f32, b: f32, c: f32, d: f32) -> f32:
      return c if a < b else d

This would effectively encode all the information you are encoding in the class and it is more akin to how we externalized instructions definitions. There is the issue however of how to make sure these externs' arguments are dynamically typed which was the case before. We might want to allow these externs to be overloaded to avoid having to write on per-type or maybe this is not an issue because it is the same problem for instruction. We might though need to allow them to be rewritten modulo precisions. e.g. swap_builtin(proc, proc.find("select_R(_)"), select_f32) or something like that.

[yamaguchi1024]

I like this interface!! It's like we're adding Exo type annotation (e.g., f32) to Python function, but the @extern function body is in Python.

[yamaguchi1024]

@SamirDroubi I think @extern makes sense and we should do it at some point, but I don't have an energy to do it now so I'll just move the typechecking into the compiler and parse the proc's externs instead of using call_depth