Initial experimentation with a -sMEMORY64=1 wasm build (full 64-bit). This compiles...

Initial experimentation with a -sMEMORY64=1 wasm build (full 64-bit). This compiles but does not pass tests due to friction between BigInt and Number types (e.g. Number(null)===0 but BigInt(null) throws, many functions are fussy about which of those types they'll take, and we cannot simply mix and match the two types transparently (1n+1 is not legal (but 1n>=1 is), so we can no longer do pointer arithmatic without hoop-jumping)). The library bootstraps but it's failing early on in tests due to this friction.

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