src: v8 api upgrade wip
string_bytes: stop using String::AsciiValue

Debug builds of V8 now actively check that the string only contains
ASCII characters (i.e. doesn't contain bytes with the high bit set.)
src: abort() on fatal error in debug mode

abort() generates a nice core dump and makes it easy to catch fatal
errors in one's debugger of choice.
src: enable native v8 typed arrays
tools: add check-imports.sh script