Import bitwise expression definitions

[qualterz]

There is need to import bitwise flag definitions: https://github.com/bitcoin-core/secp256k1/blob/01b819a8c7d485fdd3f024d77273f5769d75b2d3/include/secp256k1.h#L191-L214

Currently, any expressions are ignored:

crystal_lib/src/crystal_lib/prefix_importer.cr

Line 21 in c3012aa

return unless value.is_a?(Crystal::NumberLiteral)

[qualterz]

The naive solution I found is to add Crystal::Expressions to the allowed types:

return unless value.is_a?(Crystal::NumberLiteral | Crystal::Expressions)