Construct a BigInt from a decimal or hexadecimal string.
The number must be in the form of a D decimal or hex literal:
It may have a leading + or - sign; followed by "0x" if hexadecimal.
Underscores are permitted.
BUG: Should throw a IllegalArgumentException/ConvError if invalid character found
Construct a BigInt from a decimal or hexadecimal string. The number must be in the form of a D decimal or hex literal: It may have a leading + or - sign; followed by "0x" if hexadecimal. Underscores are permitted. BUG: Should throw a IllegalArgumentException/ConvError if invalid character found