Allow for static values as array sizes.

[m-kurtenacker]

This applies to both array types [T * n], as well as RepeatArrayExpr [E; n]. In both cases, n must be defined as static n = ..., or artic might fail.

There are three parts to this:

• Extend the ast and parser to accept ast::Path as a size, in place of a literal.
• Extend the binder to bind those paths to the static declaration.
• During type checking, replace the ast::Path with the static size taken from the declaration.
  This last step will replace the size member of the RepeatArrayExpr and the SizedArrayType, so the emitter does not require any substantial changes.

The way type inference is implemented should allow modules to be used when defining the size of an array, but addressing struct members will fail with an assert during type inference right now.