Code is now generated in the "correct" structure

THE CODE NO LONGER COMPILES!!

The ParserGen msbuild targets expects the generated code to live under Gen/Psi/Language/ and Gen/Parsing/Language. As part of the process, it removes anything under Gen, then re-adds anything in Gen/Psi/FSharp and Gen/Parsing/FSharp.

Unfortunately, all of the files have errors, so the code no longer compiles.
Moving implementations to conventional namespaces
Added visitor pattern methods to IFSharpTreeNode
Removed all semantic directives from fsharp.psi

Semantic interfaces are no longer supported by ReSharper - the semantic
interfaces merged with the main parse tree several years ago. Reduces
compile errors.
Using XXX_KEYWORD instead of KEYWORD_XXX

Use the name of the keyword token defined in tokens.xml
Replaced KEYWORD_XX_OP with symbol token

Reducing compilation errors
Use IFSharpIdentifier instead of IIdentifierNode

Further reducing compilation errors
Implemented visitor methods in FSharpTokenBase
Fixing up FSharpFile generated code
Added empty expression base classes
Deleted empty TreeNodeVisitor
Remove custom implementations flag

Until such time as we need to derive from the normal implementation,
let's just stick to the original
Added empty base classes

Working towards a clean compile, but might be better to remove and add
back if/when we need them
Fixing up unresolved tokens

Removed OPERATOR_ tokens. Will add them back in if/when we need them.
Added CARET token, by hand (but updated tokens.xml too)
Added missing error message symbols
Removed generated file
Tweaked empty constants to fix code gen

Changed emptyList, emptyAray and unit to be private nonterminals (so
they don't appear as types in the parse tree, and the rules are pulled
up into rule usage). This fixes the code gen for constantExpression and
constantPattern, which both expose a value that is either a constant
leaf token, or one of these empty types. Not sure this is right (might
be better to introduce a private aux composite to hold all of these
things, and still keep the code gen happy).

On the plus side. Clean compile!