Finch 2.1.0

Finch source files now end in .fch instead of .fn, which is already used by other tools (for example Fennel).

• import name looks for name.fch.
• If only an old name.fn is there, the compiler says so and tells you to rename it.
• All examples, tests, the self-hosted compiler (boot/*.fch) and the docs use the new extension.

To update your own files: for f in *.fn; do mv "$f" "${f%.fn}.fch"; done