Pool temporary D objects

[cristaloleg]

During encoding we've a few places where we allocate a new D for short period of time. Such allocations are harmless but we can get rid of them by allocating a D from sync.Pool.

As a result we should have zero-alloc encoding (in terms of D allocs, buffer grow or reflect is a separate topic).

Also: M.AsD() D should have an unexported version where we encode M into a given D.