Arrays

[griwes]

Implement arrays, including statically and dynamically sized ones. Dependently sized ones are in plans somewhere after #29, but let's ignore that for now except in this comment.

Mildly depends on #18 (for forming the array types); depends on #33. For sanity of the full feature, needs to understand how #21 works.

• RFC and specification
• parsing - mostly uses existing constructs, but possibly also want designated initializers for array elements?
• analysis
• codegen
• tests