Split-dwarf support

[dschuff]

Currently Binaryen supports rewriting DWARF information embedded in custom sections in a finally-linked wasm binary. LLVM recently added support for split-dwarf (i.e. putting some of the debug info in .dwo files) and creating dwarf packages (i.e. combining the .dwo files into a .dwp file).
Currently our preferred debugging workflow is to use the correct options to avoid using Binaryen at all (i.e. enable wasm-bigint and linktime-optlevel at 1 or 0); this makes linking much faster. But when using ASYNCIFY or other features which require Binaryen, this isn't an option.
If split-dwarf ends up being a recommended or preferred way of creating debug info (which currently seems likely to happen?) then Binaryen should support updating split-dwarf info in dwo and/or dwp form as well as fully-linked.

This should not be as involved as e.g. supporting object files (there are no relocations), but will be a bit of work, as some of the fields are interpreted differently in split-dwarf mode (and of course Binaryen will have to open other files to find the debug info)