Regex: Implement compiler for regular expression patterns

Originally, this developer wanted to (ab)use the PikeVM with a
hand-crafted program and an added "callback" opcode to parse the regular
expressions.

However, this turned out to be completely unnecessary: there are no
ambiguities in regular expression patterns, so there is no need to do
anything else than parse the pattern, one character at a time, into a
nested expression that then knows how to write itself into a program for
the PikeVM.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Regex: introduce debug mode

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Regex: implement ? and * operators

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Regex: Implement reluctant '?', '*' and '+'

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Regex: add support for character classes

Now we support regular expression patterns a la '[0-9]'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
TO-UNDO: makefile target for Regex