v2.0.0

Breaking API change!

• Fixed FROUND, was leaving result on data stack instead of float stack, #69

Other changes

• Added standard version numbering, eg. "2.0.0"
• Add BYE-CODE variable, which will be returned to the shell when pForth exits.
• Set BYE-CODE when a test fails to simplify continuous integration tests.
• Add ANS structure support
• Add [DEFINED] and [UNDEFINED]
• Implement MSEC using usleep() to avoid busy wait.
• Added VERSION_CODE for software version checks.
• Added S"
• Terminal is unbuffered on posix systems
• Added CMAKE build, (thanks Robin Rowe)
• Improve unix/Makefile, moved to "platforms" folder
• Added GitHub actions for CI
• Add compiler warnings about precision loss.
• Improve 64-bit CELL support.
• Allow header and code size to be more easily controlled.
• Fixed definition of PF_DEFAULT_HEADER_SIZE
• Change license to 0BSD
• Added privatize to history.fth