Fixes for C and C++ functions and namespaces (#1722)

* Fix lexing of function names

This fixes #1561.

Add a keywords state that matches inside and outside functions for
keywords.

Before this, when a keyword would appear the lexer would go to the
statements state, in which functions were not matched.

* Add tests for lexing of function names

* Unbreak previous tests

* Allow namespaced names in function statements

Add a second identifiers regex that matces all the previous identifiers
and also '::'.

I took the decision to create a second identifiers regex with '::'
inside, simply because using the old identifiers regex would hurt
performance massively on every solution I tried to craft.

* Add tests for namespaced names in functions

* Unbreak previous tests

* Add support for namespaces in namespace declarations

Add a namespace state that is entered each time the namespace keyword
is matched and lexes all name matches as namespaces.

Cases this approach doesn't cover:
* Namespaces in using declarations.
* Namespaces that prefix names in random code.
Unfortunately, in both of these cases the names before and after '::'
are not always namespaces.

* Add tests for namespace declartions

* Unbreak previous tests

* Tidy functions regex

Remove group nesting that became unneeded after fc56ab8 (the last big
refactor).

* Remove f string usage I introduced by mistake

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