Add fixed-point decimal type to tarantool core.
Adapt decNumber floating-point decimal library for the purpose, write a
small wrapper and add unit tests.

A new decimal type is an alias for decNumber numbers from the decNumber
library.
Arithmetic operations (+, -, *, /) and some mathematic functions
(ln, log10, exp, pow, sqrt) are available together with methods to
pack and unpack decimal to and from its packed representation (useful
for serialization).

We introduce a single context for all the arithmetic operations
on decimals, which enforces both number precision and scale to be
in range [0, 38]. NaNs and Infinities are restricted.

Part of #692