Changes named sub-expressions to use different hashing scheme: high order bit is now always set to clashes between hashes and indexes don't happen until 2^30 or 2^62 sub-expressions in 32 and 64 bit code respectively.
Changes bitmask of seen sub-expressions to use dynamic storage for sub-expression indexes above 64.
Adds tests for the above.
Fixes https://github.com/boostorg/regex/issues/75.
