[Core] Allow passing general objects by value to kernels

[noelchalmers]

Description

Title says it all. tests/src/core/kernel.cpp shows the intended usage, and tests/src/c/kernel.cpp shows the equivalent C API usage (C needs the argument wrapped in an occaStruct object).

[codecov]

Codecov Report

Merging #676 (b674a1e) into development (cbdd074) will increase coverage by 0.00%.
The diff coverage is 63.63%.

❗ Current head b674a1e differs from pull request most recent head 9575900. Consider uploading reports for the commit 9575900 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           development     #676   +/-   ##
============================================
  Coverage        76.23%   76.23%           
============================================
  Files              291      291           
  Lines            18798    18807    +9     
============================================
+ Hits             14330    14337    +7     
- Misses            4468     4470    +2     

Impacted Files	Coverage Δ
src/core/kernelArg.cpp	79.48% <55.55%> (-1.43%)	⬇️
include/occa/core/kernelArg.hpp	47.36% <100.00%> (+6.19%)	⬆️

... and 3 files with indirect coverage changes