127. BigDecimal subtraction

This is a follow-up on 126. BigDecimal addition

In this problem, you are asked to implement the subtraction of two decimals with arbitrary digits.

subtract('-999999999999999999', '-1')// '-999999999999999998'subtract(  '-999999999999999999.999999999999999999999999999999',   '1.0000000000000000000000000001')// '-1000000000000000001.000000000000000000000000000099'subtract(  '999999999999999999.999999999999999999999999999999',   '-1.000000000000000000000000000001')// '1000000000000000001'

1. This problem covers 77. implement BigInt subtraction with sign.
2. trailing zeroes in the result should be removed.