Add sourcemap support to wasm binary format

[hhugo]

What would it take to support sourcemap with the binary format ?

Not that much, I guess. We need to replicate what is done in Js_output. So, push a mapping for each Event, plus one at the end of each function (after the byte 0x0B) to avoid that an events bleeds over the next function. There is a single line; the column is the position in the binary file.
Plus some plumbing to write the source map to a temporary file and pass the file name to binaryen.

Originally posted by @vouillon in #2000 (comment)