In current pytest, the same expression is matched against all items. But
it is re-parsed for every match.
Add support for "compiling" an expression and reusing the result. Errors
may only occur during compilation.
This is done by parsing the expression into a Python `ast.Expression`,
then `compile()`ing it into a code object. Evaluation is then done using
`eval()`.
Note: historically we used to use `eval` directly on the user input --
this is not the case here, the expression is entirely under our control
according to our grammar, we just JIT-compile it to Python as a
(completely safe) optimization.
622c4ce02e