This new target focuses on fuzzing the actual descriptor parsing logic
by not requiring the fuzzer to produce valid keys (nor a valid checksum
for that matter).
This should make it much more efficient to find bugs we could introduce
moving forward.
Using a character as a marker (here '%') to be able to search and
replace in the string without having to mock the actual descriptor
parsing logic was an insight from Pieter Wuille.
This commit effectively moves the definition of these constants
out of the chainparamsbase to their own file.
Using the ChainType enums provides better type safety compared to
passing around strings.
The commit is part of an ongoing effort to decouple the libbitcoinkernel
library from the ArgsManager and other functionality that should not be
part of the kernel library.
* Use SECP256K1_CONTEXT_NONE when creating signing context, as
SECP256K1_CONTEXT_SIGN is deprecated and unnecessary.
* Use secp256k1_static_context where applicable.
