C domain rewrite

[jakobandersen]

Subject: essentially a rewrite of the C domain to fix issues and enable further development

Feature or Bugfix

• Feature
• Bugfix
• Refactoring

Purpose

The original implementation of the C domain relied on regex matching to parse signatures. While it works for many cases, there is a limit. This rewrite is in the same style as the C++ domain, and some details are shared between the two domains. It is naturally a large breaking change, but I believe it is necessary for furture improvement of the domain. In order to make some simple kind of judgement of the changes I have updated the cpython documentation as best as possible: https://github.com/jakobandersen/cpython/tree/doc_fixes

• xref lookup now respects the scope that it was declared in.
• Anonymous entities can now be declared with appropriate lookup rules.
• The c:type directive seemed to have been a catch-all for many signatures which made it difficult to parse and generate appropriate output. It has been changed to be much stricter and only allow either a name or a typedef-like declaration.
• There are new directives for structs, unions, enums, and enumerators. Lookup rules for enumerators are as expected from the language.
• New xref roles has been added in conjunction with the directives. (The issue from py domain: All :py:* roles can refer python objects even if objtype not matched. #7243 is not yet fixed yet)
• An c:expr role has been added to typeset a type of an expression in text. In particular while one before could write ... a :c:type:`int` and then ... or ... a :c:type:`char*` and then ... this will now trigger an error as the c:type strictly is for cross-referencing. Instead one should use the c:expr role.
• This PR incidentally reverts PR c domain: Generate node_id for objects in the right way #7269 (sorry), except the removal of the signode['names'].append(targetname) thing. However, the ID generation is now guaranteed to produce different IDs for different declarations, and they are produced from parsed input (i.e., no arbitrary code injection).

Detail

• Fixes Structure members break, when accompanied with brackets #1539
• Fixes Using regular expressions for c function prototypes #2377

[tk0miya]

Unfortuantely, I don't have time to understand the new implementation of the C domain. But it would go well because you're author :-) +1 for merging.