natives/unicode: Update signature of to.
natives/unicode: Update signature of to.

In Go 1.11, the responsibility of function to has been increased to
additionally report whether caseRange contained a mapping for r.
Update our modified copy of it to fix the build.

Fixes:

	$ gopherjs build unicode
	unicode/letter.go:248:9: cannot assign 1 values to 2 variables
	unicode/letter.go:305:20: cannot initialize 2 variables with 1 values
	unicode/letter.go:296:20: cannot initialize 2 variables with 1 values
	unicode/letter.go:287:20: cannot initialize 2 variables with 1 values

Follows golang/go@05f8b44d5edc2960eff106e5e780cf83535d0533.

gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs