`BigDecimal#to_f` is slow when it has large n_significant_digits

[tompng]

BigDecimal#to_f is accurate, it returns exact nearest float value, but may be extremely slow in some case.

irb(main):003> x = BigDecimal(1).div(7, 10000); 10000.times{(x - it).to_f}
processing time: 1.746866s
=> 10000
irb(main):004> x = BigDecimal(1).div(7, 10000); 10000.times{(x - it).mult(1, 20).to_f}
processing time: 0.053746s

If BigDecimal#to_f is optimized, we can remove workaround like #514

[byroot]

I looked a bit at this, and it seems that after #519 that particular slowness is no more?

On my machine:

before:

 1.96 real         1.58 user         0.02 sys

after (with reverting 3bf735f):

 0.64 real         0.34 user         0.02 sys

Now most of the profile is VP_ulltoa and memcpy in VpToString, with a little bit of ruby_strtod.

https://share.firefox.dev/4sVGaHo

If we want to optimize this further, JSON now use a faster algorithm: ruby/json#767, but it's under MIT

[tompng]

Looks like it is basically resolved.
Reverted the workaround for to_f(#514) in #522