don’t actually need to define variants first for `char_to_unified`
because unlike `Scanner.bscan/1`, this is matching against an entire
binary rather than just head, so no need to worry about similar out of
order issues.

One or all of these emoji #⃣ #️⃣ *⃣ were causing:
== Compilation error on file lib/exmoji/scanner.ex ==
** (Regex.CompileError) nothing to repeat at position 5795
    (elixir) lib/regex.ex:142: Regex.compile!/2
    lib/exmoji/scanner.ex:43: (module)
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6

Prior to this commit exmoji used the old `\x{H*}` format for unicode
escape sequences, which generates warnings with Elixir 1.3. This commit
updates those escape sequences to use the preferred `\xHH` and `\uHHHH`
formats.