Match arbitrary bytes

[nitely]

fixes #108

this needs some refactor and tests

let flags = {regexArbitraryBytes}
doAssert match("\xff", re2(r"\xff", flags))
doAssert replace("\xff", re2(r"\xff", flags), "abc") == "abc"

TODO:

• [x] change the parser so the pattern is iterated as chars instead of runes when setting the flag. So re2("弢", flags) matches "弢" and arbitrary bytes match arbitrary bytes. Also, runes() expect valid utf8 so this is the only way.
• [x] tests
• [x] docs