Left factor parser for function types

[Gabriella439]

Fixes #108

This gives a massive (~30x) parsing performance improvement for the benchmark
code from the above issue:

benchmarking Issue #108/Text
time                 169.9 ms   (167.6 ms .. 172.8 ms)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 174.8 ms   (172.6 ms .. 177.3 ms)
std dev              3.525 ms   (2.008 ms .. 5.131 ms)
variance introduced by outliers: 12% (moderately inflated)

After:

time                 5.860 ms   (5.826 ms .. 5.904 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 5.921 ms   (5.902 ms .. 5.939 ms)
std dev              56.42 μs   (46.69 μs .. 67.52 μs)

The root cause was that the parser for function types was introducing
excessive backtracking. This led to parsing performance being exponential
in the number of atomic operatorExpressions.

This change left-factors the expression parser by gutting
annotatedExpression. Specifically, this moves the logic for parsing unannotated
List/Optional/merge into expression and then consolidates the
remaining logic for parsing an ordinary Annot into expression, applying
a fixup for unannotated List/Optional/merge expressions to tag them with
their annotation.

[Gabriella439]

Note that still needs a little cleanup before merging because it discards the Noted constructors from type annotations. I will polish this a bit more later tonight

[f-f]

I can confirm that this also fixes #580 🎉

Running time with this branch is ~1s, which I'd consider good enough.

Thanks for the good work @Gabriel439 and @phadej 👏