msgpack_ext: add string convertion for decimal datetime interval

[babyTsakhes]

msgpack_ext: add string() for decimal

Added a decimal type conversion function to a string, added tests for this function. Fixed name of function. Add test for decimal convertion. I tried to write an optimized version, but there is a problem with values when working with BCD, insignificant zeros after the decimal point are lost.

Added #322