Support user defined precision of Decimal strings

[ear7h]

Currently, Decimal#to_s is hardcoded to support 60 digits of precision.

We should do something like (ruby)[https://ruby-doc.org/stdlib-2.2.0/libdoc/bigdecimal/rdoc/BigDecimal.html#method-i-to_s] which allows the user to pass a format string to the to_s method.
#451 for context
cc @hachi8833

[hachi8833]

Yes, that should be improved as you pointed out. I'll look into it.