Add Enumerator support to `String#each_byte`.
`String#each_byte` is not defined in ISO Ruby but it is implemented in
the core mruby because it's useful.
@@ -167,6 +167,7 @@ class String
##
# Call the given block for each byte of +self+.
def each_byte(&block)
+ return to_enum(:each_byte, &block) unless block
bytes = self.bytes
pos = 0
while pos < bytes.size
