Improve handling of strings

• String constants are now defined with `constexpr´. Because these are
(associated at runtime with) `char const*const´ values, they can be
compared more‐or‐less like the old enum values used to be; because
those pointers point to actual strings, the code for processing them
and serializing them is simplified quite a bit. A few arrays give the
list of available strings; these are ⹐not⹑ (cannot be) `constexpr´s
because while the strings themselves are known at compile time, the
pointers which point to them cannot be. Instead, they are
`static const*const´ arrays; the `static´ keyword keeps their
visibility internal.

⋯ Exceptionally, `cgirls_mtype_any´ is defined as `nullptr´ rather
  than a string of zero length; handling this should always be a
  special case.

• Most of the verbs have been commented out to reduce the amount of
  code needed for an initial working implementation.

• The path·info parsing code has been refactored a bit, making use of a
  new function, `cgirls_gobblepath´ to encapsulate the task of reading
  up thru the next slash. The serialization code has also been
  refactored here and there for tidiness.

• Some comments in `request.c´ used spaces instead of tabs. Whoops!

Note that Clang only supports `constexpr´ in version 19 and later.