fix: tweak encode::Context with future backwards compatibility in mind

- All fields are private and are accessed via getter methods;
- No `Context::new` function, as there's no good set of parameters the
  users are most likely to set. Instead the builder-style methods will
  be used to set context parameters;
- `Context::style` renamed to `Context::struct_style` to avoid confusion;
- `EncodeStyle` renamed `StructStyle` to avoid confusion;