BF functios can now be declared and called from AbleScript code.
Function parameters supplied from AbleScript are serialized manually
into byte sequences using the `BfWriter` trait, and are available from
BF code using the input operator, `,`.
At the moment, BF functios simply write output to stdout, and are
therefore incapable of communicating results to the rest of the
program; this might change in the future if we can get something close
to pass-by-reference working.