The wasm function index space consists of the index space of the
imported functions concatenated with the index space of the defined
functions. When iterating through function definitions, it's necessary
to adjust the index when a function-index-space index is needed.
This begins reorganizing how translation and compilation occur, and
setting up infrastructure for imports/exports and relocations. It
splits parts out of StandaloneRuntime, forming Module, Compilation,
and Instance structs, which can be used more independently.
It also simplifies the command-line interface, in a step towards
making simple tools that just expose the functionality of the
libraries.
