Add chance to emit just one block from janet-emit

[pepe]

This commit is the last part of cgen improvements on the side of the emitting code with the help of Janet. It adds the option where a tuple with the first member of type symbol cgen emits as one block.

I guess that would be it on this front. If you agree, I would like to add documentation for the .cgen usage with jpm.

[bakpakin]

LGTM

[pepe]

Thank you. I will start with the doc then.