[Core] Adding hostMalloc API

[noelchalmers]

Description

This PR implements a new API, hostMalloc which lets users malloc occa::memory regions in pinned host memory, if the backend device supports this, i.e. in CUDA, HIP, and somewhat OpenCL.

Edit: After consideration, we don't need to introduce a separate device::hostMalloc API for a functionality that is only present on some backends. We'll instead trigger the hostMalloc behavior by passing a "{'host': true}" prop in the device::malloc.

This PR also somewhat changes how the device/pinned host pointers are managed in backends in order to let users obtain the correct underlying pointer via the memory::ptr() method automatically, rather than having to pass a particular occa::properties setting. Users can also directly pass these hostMalloc'd buffers into kernels since GPU devices can directly read & write pinned host memory regions*. Finally, the correct copyTo/copyFrom behaviors are determined automatically based on the location of the underlying pointers.

Some examples of the resulting API change in user applications can be seen in this draft PR: paranumal/libparanumal#28

*The OpenCL spec unfortunately does not explicitly guarantee that GPU kernels will directly access pinned host memory. For now, users will continue to have to explicitly copy back the contents of hostMalloc'd buffers in order to guarantee coherency. Potentially, OCCA could also expose some kind of sync wherein a pinned host buffer would be re-mapped.

P.S. I have no idea if there is a similar functionality in Metal. If someone wants to take a look at the corresponding tweaks in the Metal backend, that would be greatly appreciated.

[codecov]

Codecov Report

Merging #376 (21534c4) into main (83194cc) will decrease coverage by 0.13%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   75.29%   75.15%   -0.14%     
==========================================
  Files         254      254              
  Lines       19438    19416      -22     
==========================================
- Hits        14635    14593      -42     
- Misses       4803     4823      +20     

Impacted Files	Coverage Δ
include/occa/core/memory.hpp	100.00% <ø> (ø)
include/occa/core/memory.tpp	100.00% <ø> (ø)
src/core/memory.cpp	71.06% <50.00%> (+0.69%)	⬆️
src/c/memory.cpp	100.00% <100.00%> (ø)
src/occa/internal/core/memory.cpp	100.00% <100.00%> (ø)
include/occa/types/json.tpp	41.66% <0.00%> (-28.34%)	⬇️
include/occa/types/json.hpp	66.50% <0.00%> (-10.68%)	⬇️
src/occa/internal/lang/specialMacros.cpp	58.62% <0.00%> (-0.87%)	⬇️
src/occa/internal/utils/sys.cpp	71.88% <0.00%> (-0.13%)	⬇️
include/occa/core/base.tpp	100.00% <0.00%> (ø)

[dmed256]

Thanks for adding this! I'll take a look at it this weekend, it's a busy week sorry 😞

[dmed256]

LGTM, thank you so much for making these changes! I made a few styling review suggestions so I could batch commit them before merging.