Add script/string roundtrip fuzz tests

[sipa]

Builds on top of #105.

This adds script and string roundtrip fuzz tests, all in the same source file (reusing some logic), and unified the names of the tests:

• miniscript_stable (the old miniscript_random)
• miniscript_smart (the old miniscript_random_smart)
• miniscript_script (round-trip from/to script)
• miniscript_string (round-trip from/to string)

[sipa]

Rebased on new #105.

[sipa]

Rebased on top of merged #105, and also addressed:

• Partially reverted to FUZZ_TARGET_INIT again, to make sure initializations aren't executed when not needed.
• s/str/str2/ in the string test

[darosior]

ACK c65113d0791da170139ab4dc2cb66394c94ddcf0