Switches to using asm incbin to embed the V8 snapshot instead of
outputing C code and then compiling it.
Compile time for from_snapshot.o goes from 44s to 1s.
Move rust.gni and deno.gni into build_extra/
Removes rust_library which was only an action. This instead defines
rust_component, which is an action plus a gn "component" target
to expose the resulting object file. This simplifies link code in
rust.gni.
Support rust modules that can be linked into C++.
