Tarantool supports decimal type since version 2.2.1 [1]. This patch
introduced the support of Tarantool decimal type in msgpack decoders and
encoders. The Tarantool decimal type is mapped to the native Python
decimal.Decimal type.

Tarantool decimal numbers have 38 digits of precision, that is, the
total number of digits before and after the decimal point can be 38 [2].
If there are more digits arter the decimal point, the precision is lost.
If there are more digits before the decimal point, error is thrown. In
fact, there is also an exceptional case: if decimal starts with `0.`,
38 digits after the decimal point are supported without the loss of
precision. msgpack encoder checks if everything is alright. If number is
not a valid Tarantool decimal, the error is raised. If precision will be
lost on conversion, warning is issued.

Any Tarantool decimal could be converted to a Python decimal without the
loss of precision. Python decimals have its own user alterable precision
(defaulting to 28 places), but it's related only to arithmetic
operations: we can allocate 38-placed decimal disregarding of what
decimal module configuration is used [3].

1. tarantool/tarantool#692
2. https://www.tarantool.io/ru/doc/latest/reference/reference_lua/decimal/
3. https://docs.python.org/3/library/decimal.html

Closed #203