Fix bytes type

[arg0d]

Type::Bytes produces incorrect code in the following situations:

• u8 is not referenced anywhere else in the definition file. This causes undefined symbol FfiConverterUint8.
• Vec<u8> is referenced somewhere in the definition file. This causes duplicate symbol definition for FfiConverterSequenceUint8.

To fix this, replace Type::Bytes with Type::Sequence<u8> in known types set before rendering Type.cs. This ensures that both FFI converters FfiConverterSequenceUInt8 and FfiConverterUInt8 will be rendered exactly once in Types.cs.

Fixes #49.

[arg0d]

The separate tests ensures that bytes type is tested in isolation without Vec<u8 and u8 types being used in UDL. Testing reverse itself is not necessary, just have to make sure the code compiles. I will move the comment from UDL file to test file.