* Rename `lower.{h,cpp}` to `ast-legalize.{h,cpp}`

This pass isn't really performing lowering akin to `lower-to-ir.{h,cpp}` so the file name is misleading.
By renaming this pass we emphasize its role as an AST-related pass.

Also update the comment at the top of `ast-legalize.h` to reflect the intended purpose of this pass in a world where we have the IR up and running.

* Allow `import` as an alias for `__import`

The use of double underscores to mark our new syntax has so far had two purposes:

1. It helps identify syntax that isn't meant to be exposed to users in its current form (e.g., `__generic` gets a double underscore because we want users to have a more pleasant surface syntax for generics that they write). This rationale doesn't apply to `__import`, which is a major language feature that users need to interact with.

2. It helps avoid the problem where the compiler treats something as a keyword that isn't supposed to be reserved in HLSL/GLSL and so causes existing user code to fail to parse (e.g., when the user tries to write a function called `import`). This no longer matters because we look up almost all of our keywords using the existing lexical scoping in the language (so the user can shadow almost any keyword with a local declaration).

So, neither of the original two reasons applies to `__import`, and it makes sense to expose it as `import`.
Doing so is a one-line change.