Constructs a BigDecimal from a string containing an optional minus
sign followed by a sequence of zero or more decimal digits, optionally
followed by a fraction, which consists of a decimal point followed by
zero or more decimal digits.
Returns the number of bits in the minimal two's-complement
representation of this number, *excluding* a sign bit, i.e.,
(ceil(log2(this <0 ? -this : this + 1))).