This means that the verification of the correctness of the regex is
done once, at the deserialisation/creation time, as in the Python
code. To do this, we have to change the FilterRegex type from an alias
to String to a more complex data type, and we have to create manual
read/show/eq instance (phew!).

Unittests are added which test these instances, since it's the first time I do
this manually.

An additional improvement is that we now check that regex-pcre has
been compiled correctly, per the documentation (otherwise we get
runtime errors).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>