Revert changes to NumberLiteral from #232

[stasm]

Fix #239. Revert changes from #232 which made value be a float, and added precision. Also rename value to raw for consistency with StringLiteral. I'll file another issue to discuss removing unescaping from the AST and moving it into the resolver.

[stasm]

@Pike, I'd like to land this separately from #243 to record the reverting of #232 explicitly in the history. To reduce the diff, perhaps I should drop the "rename value to raw" part, since we know that we'll change it anyways in #243?

[Pike]

Yeah, that might help, but would that make rebasing #246 hard?

[stasm]

I intended to rebase/squash #246 anyways, and it wasn't hard at all :)

[stasm]

I've updated this PR and I've renamed NumberLiteral.raw back to value, just as it was called before #232. This PR isn't a strict revert, however. I've simplified the NumberLiteral grammar production and I've kept the test which I had added in #235.