Use Clang.jl to automate C wrapper

[odow]

This PR uses Clang.jl to wrap the libnlopt.h header file.

It also gets rid of the various @eval and macro definitions. Boring and verbose is better than clever.

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Codecov Report

Attention: Patch coverage is 41.12903% with 146 lines in your changes missing coverage. Please review.

Project coverage is 57.91%. Comparing base (b860c0c) to head (49c4203).
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/libnlopt.jl	25.21%	86 Missing ⚠️
src/NLopt.jl	54.88%	60 Missing ⚠️

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Flag	BASE (b860c0c)	HEAD (49c4203)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   65.63%   57.91%   -7.73%     
==========================================
  Files           2        3       +1     
  Lines         777      948     +171     
==========================================
+ Hits          510      549      +39     
- Misses        267      399     +132     

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[odow]

This is backwards compatible because we now have two copies of the enums, and Base.converts between them.

[odow]

I think this is an improvement. As an example, it now exposes the C API necessary for #122

There's still a bunch of stuff we need to keep around for backwards compatibility, but we can continue to chip away at tidying it up.