Implement BigDecimal#_decimal_shift for internal use

[tompng]

Introduce BigDecimal#_decimal_shift(n) for internal use.
BigMath.exp and BigMath.log now uses it instead of x * BigDecimal("1e#{shift}")

- x = x * BigDecimal("1e#{-exponent}")
+ x = x._decimal_shift(-exponent)

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