Add #to_d support to BigDecimalWithNumericArgument

[leoarnold]

The bigdecimal/util library adds #to_d to several numeric types, where

numeric.to_d(*args)

is equivalent to

BigDecimal(numeric, *args)

and the performance improvement of BigDecimalWithNumericArgument
therefore equally applies here.

Also, the exception of Float with specified precision does not seem
to make sense and is therefore dropped:

Warming up --------------------------------------
   BigDecimal("1.2")   266.340k i/100ms
BigDecimal("1.2", 9)   261.542k i/100ms
  BigDecimal(1.2, 9)    44.366k i/100ms
            1.2.to_d    42.809k i/100ms
         1.2.to_d(9)    43.915k i/100ms
Calculating -------------------------------------
   BigDecimal("1.2")      2.634M (± 1.8%) i/s -     13.317M in   5.056712s
BigDecimal("1.2", 9)      2.510M (± 5.4%) i/s -     12.554M in   5.016970s
  BigDecimal(1.2, 9)    330.664k (±32.5%) i/s -      1.464M in   5.055963s
            1.2.to_d    183.891k (± 7.3%) i/s -    941.798k in   5.148850s
         1.2.to_d(9)    189.455k (± 9.4%) i/s -    966.130k in   5.149545s

Generated with:

require 'bigdecimal'
require 'bigdecimal/util'
require 'benchmark/ips'

numeric = 1.2
string = numeric.to_s
prec = rand(7..11)

Benchmark.ips do |ips|
  ips.report("BigDecimal(\"#{string}\")") { BigDecimal(string) }
  ips.report("BigDecimal(\"#{string}\", #{prec})") { BigDecimal(string, prec) }
  ips.report("BigDecimal(#{numeric}, #{prec})") { BigDecimal(numeric, prec) }
  ips.report("#{numeric}.to_d") { numeric.to_d  }
  ips.report("#{numeric}.to_d(#{prec})") { numeric.to_d(prec) }
end

---

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[koic]

Thanks!