Add pow method

[jeking3]

The original author of #9 deleted their repository. I applied the change manually.

This closes #9

[mclow]

Should this be a public constructor?

[jeking3]

I think we can leave it this way for now (private).

[jeking3]

It turns out this is no longer necessary with the removal of reciprocal().

[mclow]

Other than my one comment, this looks fine. I'd be really tempted to remove the Metrowerks 8.3 workarounds, though. That compiler was released in 2002/2003.

[codecov]

Codecov Report

Merging #21 into develop will decrease coverage by 0.26%.
The diff coverage is 58.33%.

@@             Coverage Diff             @@
##           develop      #21      +/-   ##
===========================================
- Coverage    67.52%   67.25%   -0.27%     
===========================================
  Files            1        1              
  Lines          271      281      +10     
  Branches        90       97       +7     
===========================================
+ Hits           183      189       +6     
  Misses           3        3              
- Partials        85       89       +4

Impacted Files	Coverage Δ
include/boost/rational.hpp	67.25% <58.33%> (-0.27%)	⬇️

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[swatanabe]

AMDG

On 06/14/2018 06:47 AM, James E. King III wrote: @jeking3 requested your review on: boostorg/rational#21 Add pow and reciprocal methods.

I'm somewhat unconvinced of the value of x.reciprocal() as opposed to 1/x, which I believe is more clear. In Christ, Steven Watanabe

[jeking3]

@swatanabe I'm not sure I understand what you mean by the 1/x in the boost::rational context. Without a reciprocal() call, the only way I see to do it otherwise is to call 1.0/rational_cast<double>(r), and then you are stuck with a double and no good way to get back to a rational<>IntType> again.

[swatanabe]

AMDG

On 06/14/2018 02:18 PM, James E. King III wrote: @swatanabe I'm not sure I understand what you mean by the `1/x` in the boost::rational context. Without a reciprocal() call, the only way I see to do it otherwise is to call `1.0/rational_cast<double>(r)`, and then you are stuck with a double and no good way to get back to a rational<>IntType> again.

Why would you need to use double? There's an operator/(CompatibleIntType, rational<IntType>) In Christ, Steven Watanabe

[jeking3]

I removed reciprocal() and used your suggestion, so the addition is just pow(..) now.

[jeking3]

It looks like this may have caused some issues in poorly scoped calls to pow() - see:
qtumproject/cpp-eth-qtum#31

Any suggestions? Should Boost.Spirit be more specific?
Does pow need to use SFINAE to disable itself like the operators?
I think pow is being defined in namespace boost which means all other boost libraries will pick it up if they include the header and then call an unqualified pow(...).