Instead of using core/snapshot_creator.rs, instead two crates are
introduced which allow building the snapshot during build.rs.
Rollup is removed and replaced with our own bundler. This removes
the Node build dependency. Modules in //js now use Deno-style imports
with file extensions, rather than Node style extensionless imports.
This improves incremental build time when changes are made to //js files
by about 40 seconds.
Fixes some sed errors introduced in c43cfe.
Unfortunately moving libdeno required splitting build.rs into two parts,
one for cli and one for core.
I've also removed the arm64 build - it's complicating things at this
re-org and we're not even testing it. I need to swing back to it and get
tools/test.py running for it.
