WIP - ::+ parses as Pexp_constructor

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Infix constructors in type declarations.
Pass the symbol through in (INFIXCON) expressions.
Treat cons as a special case of an infix constructor.
Accept more symbols in infix constructors.
Factor out the common subset of infixsymbolchar and symbolchar.