Add require 'bigdecimal/math' for running the isolated test
First step : BigMath.log(one,5) results in zero

* of course further triangulation on the other
  test cases is needed
Add in the original C code (from MRI) as reference

* https://github.com/ruby/ruby/blob/trunk/ext/bigdecimal/bigdecimal.c#L2777-2937
Raise Math::DomainError on Complex argument
Remove the Complex case from the C example
Raise ArgumentError on non integer precision
Raise ArgumentError on zero or negative precision
Raise ArgumentError if precision is nil

* was already covered in the check for precision.is_a?(Integer)
Return postive INFINITY for x = BigDecimal::INFINITY
Return a BigDecimal::NAN if arg is (Float|BigDecimal)::NAN

* needed separate treatment for Float and BigDecimal NAN
* test for float NAN was not present in MRI implementation
Return Math::DomainError if arg zero or negative
Raise ArgumentError for nil, and non numeric argument

* e.g. using "foo" as an absurd example
Add zero, one, two as private methods

* of course as module methods, they are not private ...
* so, not entirely sure if this is OK (it expands the module API
  of the BigMath module ...)
Remove some more C code from the guard code
Implement Arc Tang Hyperbolicus series for ln(x)

* still needs the scaling for very large and small values
* still needs more testing to validate the scenario of
  extreme precision (only manually tested to 15 digits
  precision now).
Remove the manual 2.718... from the code
Use BigDecimal#div and #mult to control the precision

* using BigDecimal / BigDecimal yields a 200 precision result
  and then z * z even a 400 digits precision result; this
  made the calculation absurdly slow (many seconds for one
  calculation).
* using BigDecimal#div and #mult allows explicit setting of
  the precision (like in the C code, div2 and mult2)
* now these tests (that are currently activated) pass in 0.24 seconds
  bin/jruby -S rake test:mri TEST=test/mri/bigdecimal/test_bigdecimal.rb
Use BigMath.E to allow running test in isolation

* BigMath.exp may be defined in another file, this then fails
  bin/jruby -S rake test:mri TEST=test/mri/bigdecimal/test_bigdecimal.rb
Add a high-precision test (50 digits BigMath.log)

* excluded the test by default since it slows down the
  tests by 0.2 seconds
Keep excluding the BigMath.log(42) for now

* performance, takes 0.4 seconds