* Fix bug when subscripting a type that must be split (#396)

The logic was creating a `PairPseudoExpr` as part of a subscript (`operator[]`) operation, but neglecting to fill in its `pairInfo` field, which led to a null-pointer crash further along.

* Allow writes to UAV textures (#416)

Work on #415

This issue is already fixed in the `v0.10.*` line, but I'm back-porting the fix to `v0.9.*`.
The issue here was that the stdlib declarations for texture types were only including the `get` accessor for subscript operations, even if the texture was write-able.

I've also included the fixes for other subscript accessors in the stdlib (notably that `OutputPatch<T>` is readable, but not writable, despite what the name seems to imply).

* Fix infinite loop in semantic parsing (#424)

The code for parsing semantics was looking for a fixed set of tokens to terminate a semantic list, rather than assuming that whenever you don't see a `:` ahead, you probably are done with semantics. This meant that you could get into an infinite loop just with simple mistakes like leaving out a `;`.

This change fixes the parser to note infinite loop in this case, and adds a test case to verify the fix.