WASM: various build cleanups and add initial infrastructure for a build which elides...

WASM: various build cleanups and add initial infrastructure for a build which elides the oo1 API and its dependents (worker1 and promiser). Sidebar: an attempt was made to move generation of the build rules to an external script, but the mixed-mode make/script was even less legible than the $(eval) indirection going on in the makefile.

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