Clean up update! code

[tkoolen]

I was running into some issues and decided to clean up update!.

Summary:

• Most importantly, avoids using pointer in the ccalls, which violates GC safety. Before this change, I was running into segfaults while trying to update! P. This may also have been partly because of https://github.com/oxfordcontrol/OSQP.jl/blob/69d25e71c6d68c043ae6437a6c45dbd902ba4bda/src/interface.jl#L325 which, if Px_idx was a vector, can make it unreachable from the point of view of the garbage collector and hence up for garbage collection (in addition to being type-unstable).
• Adds separate functions for each piece of updatable data (e.g. update_q!, update_l!, etc.) to match the C interface. Some users might prefer using these, especially since keyword args have a bit of overhead in Julia 0.6. These functions are also type stable and hence faster.
• Handles the kwargs without creating an intermediate Dict
• Changes e.g. https://github.com/oxfordcontrol/OSQP.jl/blob/69d25e71c6d68c043ae6437a6c45dbd902ba4bda/src/interface.jl#L123-L124 https://github.com/oxfordcontrol/OSQP.jl/blob/69d25e71c6d68c043ae6437a6c45dbd902ba4bda/src/interface.jl#L257 to use .= so that they work completely in-place and hence don't allocate.
• net reduction in code

[codecov-io]

Codecov Report

Merging #12 into master will decrease coverage by 0.61%. The diff coverage is 79.66%.

@@            Coverage Diff            @@
##           master     #12      +/-   ##
=========================================
- Coverage   79.91%   79.3%   -0.62%     
=========================================
  Files           4       4              
  Lines         488     488              
=========================================
- Hits          390     387       -3     
- Misses         98     101       +3

Impacted Files	Coverage Δ
src/interface.jl	68.31% <79.66%> (-1.24%)	:arrow_down:

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[bstellato]

That's great! Thank you so much for this. The garbage collector changes can give lots of problems in Julia 0.7 and it is good to fix them before it is released.

Everything looks good and the tests pass. I will merge it.