This reverts commit 086dd299d4.

!! 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.

C++20 introduces language support for coroutines, using three new
keywords and related language constructions.

* co_await is a unary operator.
* co_return introduces a statement, and works just like return or throw.
* co_yield similarly introduces a statement, but cannot stand for
  itself.

Add support for these facilities to the parser and highlighter.