In total, this PR saves ~60k.
The conditional compilation on the _Debug FFI function eliminates one
of the ToCss variants, eliminating 54.4k, as well as a bunch of other
<1k functions. Removing the public trait implementation of Debug for the
font metrics provider eliminates the last Debug impl from stylo.
MozReview-Commit-ID: nIfQ3sy4OW
This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.
Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.
Then we should make the font provider mandatory, and implement the missing stylo
bits.