> Lua doesn’t have assignment as an expression.

That's quite easy to achieve if you directly generate bytecode. See e.g. https://github.com/rochus-keller/som.

> Lua 5.1 has float as the only numeric type

It internaly differs between int and float.

> Ruby, Python and JS all need to copy x into a new VM Register when it’s used

Even the OpenSmalltalk VM is much faster than CPython, as well as V8.