crud: improvments

[oleg-jukovec]

The patchset makes a few changes:

1. It removes NewEncoder/NewDecoder from the public API.
2. It change a default tuple alias from []interface{} to interface{}. This is necessary to use a custom types with custom encoders as tuples.
3. It adds examples with a request/response processing.
4. It removes dependency from github.com/markphelps/optional. The package gives a big overhead in benchmarks.
5. It makes requests immutable. It is more suitable for Go & GC. See also: v3: make some types immutable #238
6. I removed special Result* types. They don't make much sense for crud since the response format is the same.
7. A user can specify a custom row type for Result.

I didn't forget about (remove if it is not applicable):

• Tests (see documentation for a testing package)
• Changelog (see documentation for changelog format)
• Documentation (see documentation for documentation style guide)

Related issues:

Closes #271

[DifferentialOrange]

Seems fine