Allow generation of header-only code

[Chlumsky]

Make it possible to generate parsers and serializers in inlined form that doesn't need to have its own translation unit. The user should be able to pick between the following modes:

• Header + source file (current output)
• Single header with implementation directly inlined in class definition
• Single header with implementation directly following the class definition in the same file
• Definition header + inline implementation header which is included at the bottom of the first one

[Chlumsky]

Also, for the source file option, allow specifying an API class prefix like so:

class MYMODULE_API MyParser {
    // ...