`StyleComputer::font_matching_algorithm` was creating a copy of a
`FlyString` every time a `MatchingFontCandidate` was constructed or
copied, causing millions of unnecessairy reference updates when a
lot of fonts are loaded.
While a more permanent solution would be to not load so many unused
fonts, let's do the right thing and remove the unnecessairy copies of
`FlyString`.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
