[wasm-ld] Allow importing/exporting the output module's memory with arbitrary names

This adds an `--export-memory` option to wasm-ld which allows passing
a name to give to the exported memory, and extends `--import-memory` to
allow passing a <module>,<name> pair specifying where the memory should
be imported from.

This is based on https://reviews.llvm.org/D131376, with the main
difference being that it only supports exporting memory by one name.

Differential Revision: https://reviews.llvm.org/D135898