StringLowering pass (#6271)

This extends StringGathering by replacing the gathered string globals to imported
globals. It adds a custom section with the strings that the imports are expected to
provide. It also replaces the string type with extern.

This is a complete lowering of strings, except for string operations that are a TODO.

After running this, no strings remain in the wasm, and the outside JS is expected
to provide the proper imports, which it can do by processing the JSON of the
strings in the custom section "string.consts", which looks like

["foo", "bar", ..]

That is, an array of strings, which are imported as

(import "string.const" "0" (global $string.const_foo (ref extern))) ;; foo
(import "string.const" "1" (global $string.const_bar (ref extern))) ;; bar

0 files changed, 0 insertions, 0 deletions