wasm: asyncify the dylink.0 custom section parser

This commit refactors the simple binary parser in the dyld script in
charge of parsing the dylink.0 custom section. Previously the parser
was synchronous and operated on the entire input buffer; this was
simple and easy and worked well enough when the input wasm modules are
instantly read from local filesystem.

However, when running dyld in the browser, the wasm modules are
transferred via fetch() requests. The host ghc and the browser might
not be on the same machine, so slow network uplink does need to be
considered. We only need to parse dylink.0 custom section to extract
dependency info, and dylink.0 is the very first custom section in the
wasm shared library binary payload, so the parsing process should not
require fetch() to complete and should return the parsing result asap.

Hence the refactorings in this commit: asyncify the parser, make it
only consume as many bytes as needed by invoking an async consumer
callback. The input is a readable stream from the fetch() response;
once the response is available, the async wasm compilation can start
in the background, and dylink.0 parsing shall end asap which results
in more wasm shared libraries to be loaded earlier. Profit.

(cherry picked from commit 929df0ba)