fuzz: adapt Miniscript targets to Tapscript

We introduce another global that dictates the script context under which
to operate when running the target.

For miniscript_script, just consume another byte to set the context.
This should only affect existing seeds to the extent they contain a
CHECKMULTISIG. However it would not invalidate them entirely as they may
contain a NUMEQUAL or a CHECKSIGADD, and this still exercises a bit of
the parser.

For miniscript_string, reduce the string size by one byte and use the
last byte to determine the context. This is the change that i think
would invalidate the lowest number of existing seeds.

For miniscript_stable, we don't want to invalidate any seed. Instead of
creating a new miniscript_stable_tapscript, simply run the target once
for P2WSH and once for Tapscript (with the same seed).

For miniscript_smart, consume one byte before generating a pseudo-random
node to set the context. We have less regard for seed stability for this
target anyways.

Author: darosior