AutoLayout → LayoutVisitor

[dhardy]

Replace trait AutoLayout with LayoutVisitor using RPITIT to move the temporary layout visitor into a method.

Code should be more inlinable for better optimisation and has reduced size in optimised builds, but debug builds are larger. Size of gallery (bytes):

Commit	Debug	Stripped	Release	Stripped
16bd205	248853056	37325408	23188160	14148312
c851799	248793400	37321136	23172256	14143248
c792102	248971880	37350112	23158136	14135344
f40e17d	248971224	37350112	23158088	14135344
1706ddc	249231608	37392720	23117136	14085992

Also fix docs.rs builds for kas-dylib, kas-resvg. See rust-lang/docs.rs#2359.