ast: remove necessity for 'Ω's

[DeedleFake]

It should not be necessary to specify where the EOF is expected in the grammar. This was originally a workaround, but it's now causing problems with expression evaluation. In particular, the expression parser is incapable of parsing compounds at the moment.

I've kind of started working on fixing this in the dev/omega branch, but I'm not sure what exactly I've done wrong. This might require a fairly heavy refactoring of the parser code, although I hope it doesn't.

[DeedleFake]

Depending on how this works out, this might remove the need for a separate grammar for parsing standalone expressions, which could, in turn, remove the Go 1.9 requirement.

[DeedleFake]

This will likely become basically unecessary once #53 is done.