!! Breaking change due to restructuring and renaming of scoped identifiers !!

Scoped identifier rules have been renamed to "qualified_x_identifier"
and all variants are aliased so they appear as simply
"qualified_identifier" in the syntax tree. This name better reflects the
c++ standard's language describing this concept.

Qualified identifier rules are now always the "parent" with a right
associative rule that lets them nest indefinitely. Each rule terminates
with some kind of non-scope identifier.

This commit also adds support for the 'template' keyword where required
to disambiguate dependent template names. Field accessors (., ->) can be
followed by keyword template before a template method. The scope
resolution operator can be immediately followed by the 'template'
keyword to indicate the following dependent name with '<' is actually a
template and shouldn't be parsed as a binary operator. These keywords
are required for dependent names in these contexts.

Fixes #49