objdict: Support creating dict from another dict.
py: Fix dict.copy() and low-level map/set allocation.

Two things: 1) set flags in copy properly; make mp_map_init() not be too
smart and do something with requested alloc size. Policy of using prime
numbers for alloc size is high-level policy which should be applied at
corresponding high levels. Low-level functions should just do what they're
asked to, because they don't have enough context to be smarter than that.
For example, munging with alloc size of course breaks dict copying (as
changing sizes requires rehashing).
objdict: Implement equality operator.

Sure, it's O(n^2).
tests/dict2.py: Add combined test for dict(dict) and dict equality.
tests: Fix few tests which depend on order of elements in dict.

With dict being unordered of course.