Refactor parsing of SQL comments and type annotations inside

[rbock]

Copied from #106, thanks @MeanSquaredError

A quick glance at the code shows 5 places where you do the actual regex parsing of the cpp_type annotation

https://github.com/rbock/sqlpp23/pull/110/changes#diff-2f7fe8a53788d55d3ad155b13719339bbb6541e6233bdcd34d60abda0102f14aR246
https://github.com/rbock/sqlpp23/pull/110/changes#diff-2f7fe8a53788d55d3ad155b13719339bbb6541e6233bdcd34d60abda0102f14aR247
https://github.com/rbock/sqlpp23/pull/110/changes#diff-2f7fe8a53788d55d3ad155b13719339bbb6541e6233bdcd34d60abda0102f14aR324
https://github.com/rbock/sqlpp23/pull/110/changes#diff-2f7fe8a53788d55d3ad155b13719339bbb6541e6233bdcd34d60abda0102f14aR465
https://github.com/rbock/sqlpp23/pull/110/changes#diff-2f7fe8a53788d55d3ad155b13719339bbb6541e6233bdcd34d60abda0102f14aR528

This should be cleaned up (the code duplication should be removed).