Some response fields should have more semantic types

[kstep]

Some fields in response types has types, which doesn't match field semantics.

E.g. all boolean values VK passes as 0/1 integers, so all response fields which should be bool, are u8 now.

This PR to serde may help with this issue: serde-rs/serde#198
It adds an ability to apply custom deserializers to some struct fields with attributes.
If this feature (or some similar feature) will land to serde, we can use it to avoid writing
deserializers manually (which I want to avoid at all costs, or the code will explode beyond
any manageable limits).

[kstep]

See also #54.