Type improvements

[st-pasha]

Scheduled improvements to Type class. These should be implemented for version 1.1.0.

• [x] #3138
• [ ] Type objects should be usable in [...] / type constructs;
• [ ] dt.int32 etc singletons should be replaced with dt.Type.* instances;
• [ ] Add missing Type objects into the dt. namespace: dt.date32, dt.time64, dt.void;
• [ ] Add tests in tests/types/test-type.py for using type objects in different places:
  • In Frame constructor;
  • In f[type] selector;
  • As a type cast dt.Type.int32(f.A);
  • As a type assignment DT['A'] = dt.Type.int32;
  • In function dt.as_type();
  • In fread
• [ ] Type objects should be used by fread when handling columns=callable;
• [x] #3101
• [ ] Add method .to_ctype() similar to stype.ctype property;
• [ ] Add method .to_dtype();
• [ ] Add method .to_struct() similar to stype.struct property;
• [ ] Deprecate usage of stype and ltype enum (issue a warning when using them).