Consider using indices rather than names for symbol identifiers

[sunfishcode]

A syminfo currently identifies its symbol by name. It seems like it would be simpler, more consistent with wasm encoding in general, and consistent with the ELF-like principle of not referring to symbols within the same module by name, and possibly faster to link, if it identified symbols by index.

It would also work well with #29, in case it became important to add symbol flags to local symbols, because local symbols have indices.

It would need some way to indicate the type of index: function, global, table, or memory. I expect we could either split WASM_SYMBOL_INFO into 4 codes, WASM_FUNCTION_SYMBOL_INFO and so on, or we could add external_kind bytes.

[NWilson]

Under review: https://reviews.llvm.org/D42117

[sbc100]

Could you create PR to update Linking.md accordingly?

[sbc100]

If we go ahead with this plan then this will be fixed and we will get rid of SYMBOL_INFO completely:
#38

[sbc100]

We can reopen if #38 doesn't happen.

[NWilson]

Can close now