mark: reuse compiled expression for all items in -k/-m

The previous commit made this possible, so utilize it.

Since legacy.py becomes pretty bare, I inlined it into __init__.py. I'm
not sure it's really "legacy" anyway!

Using a simple 50000 items benchmark with `--collect-only -k nomatch`:

Before (two commits ago):

   ======================== 50000 deselected in 10.31s =====================
         19129345 function calls (18275596 primitive calls) in 10.634 seconds

   Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.001    0.001    2.270    2.270 __init__.py:149(pytest_collection_modifyitems)
        1    0.036    0.036    2.270    2.270 __init__.py:104(deselect_by_keyword)
    50000    0.055    0.000    2.226    0.000 legacy.py:87(matchkeyword)

After:

   ======================== 50000 deselected in 9.37s =========================
         18029363 function calls (17175972 primitive calls) in 9.701 seconds

   Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    1.394    1.394 __init__.py:239(pytest_collection_modifyitems)
        1    0.057    0.057    1.393    1.393 __init__.py:162(deselect_by_keyword)

The matching itself can be optimized more but that's a different story.

testing/test_pytester.py

@@ -443,7 +443,7 @@ def test_testdir_subprocess_via_runpytest_arg(testdir) -> None:
 
 
 def test_unicode_args(testdir) -> None:
-    result = testdir.runpytest("-k", "💩")
+    result = testdir.runpytest("-k", "אבג")
     assert result.ret == ExitCode.NO_TESTS_COLLECTED