sql: separate VDBE memory holding positive and negative ints

As it was stated in the previous commit message, we are going to support
operations on unsigned values. Since unsigned and signed integers have
different memory representations, to provide correct results of
arithmetic operations we should be able to tell whether value is signed
or not.
This patch introduces new type of value placed in VDBE memory cell -
MEM_UInt. This flag means that value is integer and greater than zero,
hence can be fitted in range [0, 2^64 - 1]. Such approach would make
further replacing MEM_* flags with MP_ format types quite easy: during
decoding and encoding msgpack we assume that negative integers have
MP_INT type and positive - MP_UINT. We also add and refactor several
auxiliary helpers to operate on integers. Note that current changes
don't add ability to operate on unsigned integers - it is still
unavailable.

Needed for #3810
Needed for #4015