Introduce clcall for ccall-like kernel invocation.

[maleadt]

By passing the tuple type, OpenCL.jl can do proper argument conversions, and doesn't rely on the type of the arguments happening to be correct (which can be fragile, e.g., when using literals). In addition, this makes it possible to pass CLArrays to kernels expecting a pointer, without having to explicitly access the underlying buffer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.24%. Comparing base (3204e6a) to head (1df2bef).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   76.96%   77.24%   +0.27%     
==========================================
  Files           3        3              
  Lines         165      167       +2     
==========================================
+ Hits          127      129       +2     
  Misses         38       38              

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