F#: Implement comment nesting

[rdipardo]

F# inherits nested comments from OCaml. Unlike other ancestral constructs purged from the recent standard, this is a core syntactic feature.

While LexFSharp drew most of its patterns from LexCaml, I wasn't skilled enough then or now to adapt the latter's implementation of nested comments with enumerated styles, and adding more lexical classes now would unjustifiably clutter the interface.

This patch implements lexing and folding of nested stream comments using line state:

0001-LexFSharp-Implement-Comment-Nesting.diff.txt

[rdipardo]

Test files are in this patch:

Issue93-Test-Comment-Nesting.diff.txt

[nyamatongwe]

Committed.