Implement BigDecimal#_decimal_shift for internal use

[tompng]

It can be used in sqrt calculation

BigDecimal(Integer.sqrt(BigDecimal(2)._decimal_shift(2*prec)))._decimal_shift(-prec)

irb(main):002> x = BigDecimal('1'*10000)
irb(main):003> 10000.times{x*10}
# processing time: 0.064433s
=> 10000
irb(main):004> 10000.times{x._decimal_shift 1}
# processing time: 0.041025s
=> 10000
irb(main):005> 10000.times{x/10}
# processing time: 0.287047s
=> 10000
irb(main):006> 10000.times{x._decimal_shift -1}
# processing time: 0.022237s
=> 10000