More validation performance

[markwpearce]

Refactors local variable checking to only check un-changed AstSegments for function shadowing if those segments make an assignment to a symbol that changed.

Before:

validate@local   ---------- 51.368 ops/sec
validate@0.65.26 --------- 101.435 ops/sec

After:

validate@local   --------- 286.331 ops/sec
validate@0.65.26 --------- 102.877 ops/sec

[TwitchBronBron]

Woah! That's some serious performance improvements!!! I'll test this out in our huge project to make sure it didn't break anything, but I'm super excited about this