Reduce allocations by freezing paths

[casperisfine]

Base on #310 for CI purposes.

This patch is based on an implementation detail of MRI. Pretty much all File methods call FilePathStringValue() on the passed path. This method will always dup the string unless it's already frozen. As a result freezing paths before calling `File methods will save one string allocation the vast majority of the time.

s = __FILE__
alloc = GC.stat(:total_allocated_objects)
File.exist?(s)
p GC.stat(:total_allocated_objects) - alloc # => 1

s = __FILE__.freeze
alloc = GC.stat(:total_allocated_objects)
real = File.exist?(s)
p GC.stat(:total_allocated_objects) - alloc # => 0

Of course it only really matter it loops, but Bootsnap have a few of these.