Cannot instantiate a shared memory with a custom page size

[alexcrichton]

Given this module:

(module
  (memory 1 2 shared (pagesize 1))
)

this cannot be instantiated:

$ wasmtime run -Wcustom-page-sizes foo.wat
Error: failed to run main module `foo.wat`

Caused by:
    0: failed to instantiate "foo.wat"
    1: shared memory can only be built from a static memory allocation

This is due to the fact that custom page sizes currently force a "dynamic" memory where the base pointer can change while shared memories are required to be "static" where the base pointer cannot change. Fixing this will require refactoring exactly how memories are represented internally within Wasmtime, but in the meantime I wanted to file an issue about this to have on-hand.