Clean up tooling parser around `getLiteral`

[Pike]

In reviewing #311, I found that getLiteral and getSelectorExpression are just very confusing.

For one, getLiteral doesn't do what it says, but then it also returns, in order, a ref, a ref, a literal, a ref and finally a literal again. Trying to find out what that code actually returns, and what of that's left over at which point in getSelectorExpression ... took me a while.

My ad-hoc proposal is to just rip out getLiteral entirely, and fold it in to getSelectorExpression.

I'd expect the code to be easier to read, in particular if we'd add a comment or two.

And we don't need to figure out a name for getLiteral if we'd keep it and make it do literals and non-literals in order.

[stasm]

Great suggestion. getLiteral is only used in getSelectorExpression anyways. Folding it into it will make the early returns easier to make, well, early.

P.S. Let's rename getSelectorExpression to getInlineExpression along the way, too.

[stasm]

#318 landed on zeroeight and I think we can mark this as fixed. There are more improvements which will be made possible once projectfluent/fluent#221 lands in the spec.