Add options to wat-writer

[binji]

@kanaka mentions here that having some of the sugar makes for easier debugging. It shouldn't be too much work to add options to the wat-writer to support this:

• inline function types for imported functions (not valid in text format)
• inline imports
• inline exports
• mnemonic labeling of variables (you can do this currently with the --generate-names flag, perhaps should be done by default?)
• optional expression folding
• use import/export name as debug name if no other debug name is supplied

[kanaka]

@binji yes, function types is what I was referring to.

[binji]

OK, thanks. wasm2wast already displays the inline function type for defined functions, but doesn't display them for imported functions.

[binji]

I realized it's not too hard to do expression folding, and fall back to stacky format when it's not possible: #460

[binji]

Added last feature (inline imports) here. Closing.