Apply and enforce consistent coding style using clang-format-7

[hannob]

Currently expat has no consistent coding style. See e.g. this piece of xmlparse.c:

static XML_Bool  /* only valid for root parser */
startParsing(XML_Parser parser)
{
    /* hash functions must be initialized before setContext() is called */
    if (hash_secret_salt == 0)
      hash_secret_salt = generate_hash_secret_salt(parser);
    if (ns) {
      /* implicit context only set for root parser, since child
         parsers (i.e. external entity parsers) will inherit it
      */
      return setContext(parser, implicitContext);
    }
    return XML_TRUE;
}

XML_Parser XMLCALL
XML_ParserCreate_MM(const XML_Char *encodingName,
                    const XML_Memory_Handling_Suite *memsuite,
                    const XML_Char *nameSep)
{
  return parserCreate(encodingName, memsuite, nameSep, NULL);
}

The first function indents with 4 spaces for the main function, but 2 spaces for the rest. The second function indents with 2 spaces. Similar examples can be found throughout the code.

I don't have any strong feelings about tabs vs. spaces or any other parts of coding style, but I think the style should be consistent within a project (or at least within a source file). To avoid too much unnecessary changes I'd recommend going for a coding style that's as close as possible to the already existing style.

Ideally the coding style should be documented in the form of a set of parameters for a common style checking tool (like clang-format or GNU indent) and there should be a possibility to check the whole code for compliance.

[hartwork]

True. The question is when to best do that transition because any patch we produce after will give a harder time backporting to distros.

[hartwork]

Set milestone to 2.3.1 as #220 needs to be merged for 2.3.0, first.