Immediate types

Like the previous version, this is mostly based on the 3.5 version of upstream, but the code here also incorporates immediate types; most notably, there are variants of fixnums on all platforms. Built-in symbols are also represented in ways that are completely determinable at compile-time, which should open the way to some optimisations.

The ARM and AVR versions have been tested adequately; due to circumstances, the RISC-V version has not. I hope to be able to add both RISC-V and ESP versions soon. (The build system generates them, I just don't feel comfortable publishing them without testing.)