msgpack: support decimals with negative scale

[DifferentialOrange]

Current decimal external type parser do not expect negative scale in decimal payload. Negative scale is a positive exponent. It seems that the only way to obtain positive exponent is to use E-notation since Tarantool library do not truncate trailing zeroes before decimal point:

tarantool> msgpack.encode(decimal.new('1e33')):hex()
---
- c70301d0df1c
...

tarantool> msgpack.encode(decimal.new('1000000000000000000000000000000000')):hex()
---
- c713010001000000000000000000000000000000000c
...

There are two different bugs in current implementation:

• we support only positive fixint scale and do not expect int 8 [2],
• we do not expect negative scale so positive exponent will be ignored.

This patch fixes both of them. See also [3].

1. https://github.com/tarantool/tarantool/blob/ba749e820bf0638aa3f79f266848590f9713c1cf/src/lib/core/decimal.c#L432-L450
2. https://github.com/msgpack/msgpack/blob/master/spec.md
3. https://github.com/tarantool/go-tarantool/pull/314