Rewrite the function type parser to avoid backtracking

The function type parser `functionType` (as of merging #947) uses two backtracks that are not necessary, the grammar is LL(2) [?] and can be therefore parsed without backtracking.

The biggest annoyances are that:
1. the function can also produce an atomic type (`atomicType`)
2. the vparams of a function can be:
a. missing completely (like in `=> String`)
b. `()` (see `isUnitLiteral`)
c. a single atomic type `Int` [<- this is the one we want to return in 1.]
d. k-tuple of types `(Int)`, `(Int, Bool)`, ...