Fix fast-path of frac and _decimal_shift affected by BigDecimal.limit

[tompng]

Fixes these bugs

BigDecimal.limit 3

BigDecimal(123456789)._decimal_shift(9)
=> 0.123e18 # should be 0.123456789e18

BigDecimal(0.0123456789).frac
=> 0.123e-1 # should be 0.123456789e-1

Suppress rounding in
Fast-path of _decimal_shift: shift is a multiple of BASE_FIG
Fast-path of frac: fix part is zero