base::lambda becomes just std::function and base::lambda_once becomes
base::unique_function - a move-only wrapper around std::function.
This is required because Visual C++ 2017 15.4.1 has a compiler bug
with static member variables of class templates, they may collide.
The std::function uses inheritance and virtual functions instead of
custom vtables done by static members of class templates used in
custom base::lambda implementation, so they work fine.
Now base::lambda can be copied and can wrap any immutable lambda.
For mutable lambdas there is base::lambda_once (which you're
supposed to call only once to pass data through lambda captures,
for example by using std::unique_ptr). Generally base::lambda is
passed by value and base::lambda_once is passed by rvalue reference.
All errors that lead to MTP request resending by default
error handler now can be handled differently. For example
inline bot requests are not being resent on 5XX error codes.
+ extensive use of auto keyword in MTP types handling.
John Preston