`BigDecimal(string, prec)` can allocate huge memory

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Precision passed to BigDecimal(string, prec) has no effect, but can allocate huge memory.
Just ignoring precision is better.

BigDecimal('12', 100000000000)
# processing time: 4.705712s

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And can set invalid MaxPrec This is fixed in #362. Original issue of BigDecimal('12', 100000000000) persists.

x = BigDecimal(BigDecimal._load("30:"+'1'*1000), 0)
x.precs #=> [1008, 27]