Speed up path() when passed a Path::Tiny object.

This happens a lot with libraries that take string paths but internally use and return Path::Tiny objects.

path( Path::Tiny ) is about 50% faster.
path( String ) is about 5% slower.
Check on 5.18.1 with a Macbook Pro

The extra ref $path before the ->isa check reduces the impact on path( String )
while keeping path( Path::Tiny ) about as fast.