Allow array index to be uint256

[charles-cooper]

What I did

Fix #1478 and #1483

TODO:

• tests

How I did it

This goes back to commit 4448705.
Before that, the type of the index was inferred. After that, it changed
to be the default numeric type in the language, num (now int128).

Array index types should generally be uint256.

How to verify it

Check IR of following:

xs: uint256[10]
# runtime bounds check
@public
def signed_bounds_check(ix: int128) -> uint256: 
  return self.xs[ix]

# allow uint256 index
@public
def uint256_index(ix: uint256) -> uint256: 
  return self.xs[ix]

Check this throws compile-time exception:

xs: uint256[10]
# compile-time failure
@public
def compile_time_bounds_check() -> uint256: 
  return self.xs[10]

Description for the changelog

• Allow array index to be uint256 or int128
• Fix runtime bounds checks for signed indices
• Convert runtime bounds checks into compile-time check for literal
  indices.

Cute Animal Picture

[charles-cooper]

Per discussion in #1477, I added support for both int128 and uint256 array indexes. I also fixed #1483 and converted the runtime out of bounds access check into a compile-time check in the case that the index is a literal.

[jacqueswww]

LGTM

[jacqueswww]

Add tests and we can merge :)