Problem: More functions can be used as methods.
Solution: Make a few more functions usable as a method.
64b4d73524
Note that the old-style version of Test_byteidx() was already translated
to a Lua test in 069_multibyte_formatting_spec.lua. Keep both versions,
using Test_byteidx() to mainly test the method call syntax for byteidx()
and byteidxcomp().
56b56a76e8
