rustdoc: reduce allocs in FnDecl::inner_full_print
Instead of maintaining parallel buffers for both HTML and non-HTML output, follow the idiom from the rest of format.rs that f.alternate() == true means textual output. Also, add an argument to control line wrapping explicitly.
This allows the caller to render once with textual output and no line wrapping, to decide whether line wrapping should be applied in the final HTML output.
Also, remove some format! and " ".repeat calls, and remove a dependency on calling `String::replace` to switch from newlines to spaces.
This coincidentally fixes some minor bugs where the old code was undercounting the number of characters for a declaration in text mode.
d610b0c514