Add backend support for multiple tables

[ddcc]

This patch is a derivative of #642 that only updates backend data structures and implements an internal API for multiple tables. Parsing/generation of both the binary and s-expression format are unchanged.

[kripken]

Overall I don't see anything wrong in the code. But what is the goal here? If it's code for experimentation on a side branch that sounds fine, but I don't think this is something we'd want to merge to master unless the wasm spec changed to have multiple tables.

[ddcc]

Correct me if I'm wrong, but I thought that #682 added support for multiple tables, though the MVP only supports one default table per #727?

[kripken]

I think multiple tables are something to be considered after the MVP, and it's my understanding that all post-MVP features are yet to be figured out. But I could be wrong.

[ddcc]

Is this OK to merge?

[kripken]

I still have the question from before: while the spec has just one table, I'm not sure we should have multiple table support in binaryen.

It's ok to prototype future features in binaryen. If they add no maintenance burden, the master branch can be fine. But in this case, it's a significant change, so my tendency is to keep it on a side branch.

[kripken]

No clear decision to land or not land here. Let's close this for now. Please reopen if things change or if I got it wrong.