refactor docstring generation code

[davidhewitt]

As a first step towards #2866 this is a tweak to the macro code which generates Python docstrings. This PR refactors the behaviour so that instead of always creating a concat! expression to generate a nul-terminated string, this will only happen if a Rust 1.54+ macro doc is present (e.g. #[doc = include_str!(...)]).

The default case now just collects all the #[doc] attributes into a single string.

This should make it easier to factor out the text_signature formatting, and avoids wasting compile time invoking the concat! macro when not necessary.

[davidhewitt]

bors r=mejrs

[bors]

Build succeeded:

• conclusion