Type improvements

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).