Clean up portable memory definitions

[dcodeIO]

Related: #971

This PR cleans up the portable memory definitions by, well, removing them completely. These have been of very little use anyway because there isn't a good memory story in JS. The compiler now uses bindings to Binaryen's _malloc etc. directly, which is much clearer. There's more to consider once we want to compile the compiler to Wasm, but leaving that for later.

[dcodeIO]

With WebAssembly/binaryen#2476 merged and Binaryen updated, the last commit introduces the concept of a portable d.ts.

What this does is if compiling with tsc, it will pick up the js file next to the d.ts normally, while asc ignores the js file and uses the d.ts as if it was a normal AssemblyScript file, in turn making a WebAssembly import of all the declares. As such, when compiling to WebAssembly, we can provide binaryen.js (no matter if compiled as JS or Wasm) as an import named binaryen with no need for special glue code, with the JS engine acting as a linker for Wasm exports to Wasm imports directly.