Evaluate fixed-point decimal arithmetic library

[lrettig]

Currently we're using a custom solution: https://github.com/spacemeshos/fixed

Consider switching to one of these alternatives:

• https://github.com/shopspring/decimal
• https://github.com/ericlagergren/decimal (used in https://github.com/spacemeshos/economics)
• https://github.com/cockroachdb/apd

See also http://speleotrove.com/decimal/

Related: #3408

[lrettig]

One issue with https://github.com/spacemeshos/fixed is that it's very poorly documented and hence very difficult to understand what's going on

[dshulyak]

we don't really need any floating point library

for computing weight in the tortoise fractional numbers are used, so we are not loosing any precision for computing binomial cdf - we can do that using big int with higher precision