Support forward declarations for C-like languages

[masnagam]

Output forward declarations for C-like languages:

• Definition for opaque types will not output if those are used as pointer types
• Declaration order is changed in order to solve problems of recursive type definitions

Limitations:

• No forward declaration will be output when --style=type is specified
• Forward declarations for template types in C++ are not supported

Fixes #43

[emilio]

So... I'm not sure why we need a separate set around, it seems generally this should be tracked as part of Dependencies?

Also, I don't think we should unconditionally create forward declarations for non-problematic cases as you're doing here (anything that is a pointer gets forward-declared).

[emilio]

nit: space. But also rather than having an is_ptr param, it seems this should be handled in Type::Ptr itself?

[masnagam]

So... I'm not sure why we need a separate set around, it seems generally this should be tracked as part of Dependencies?

Also, I don't think we should unconditionally create forward declarations for non-problematic cases as you're doing here (anything that is a pointer gets forward-declared).

i'll check your points and modify this PR when i have time.