Add tests for custom keyword parsing
Move l shortcut into pytest namespace
Ignore non-string function docstrings
Add evals shortcut for eval(read(...), context)
Fix put interpretation

Do not intern symbols early, but only after the value was verified, and
uniformly unquote the value before processing, if it is a quoted value.
Add tests for interpreter functions
Add test for Source.empty
Add tests for intern
Properly handle strings in intern()