Break up __pin_project_internal! into multiple rules.

By removal internal rules, this commit turns one giant macro with 47
rules into 23 macros with a small number of rules each. I find the
resulting code *much* easier to read; I found the original giant macro
quite overwhelming. The new code also needs fewer comments because the
code is clearer.

The commit also fixes some minor formatting inconsistencies, such as
always putting a space between the `!` and the `{` in a macro
invocation, and using parens instead of braces for a few matchers.