Added kernel caching support

[dpanfilyonok]

Proposed Changes

Added support of kernel caching. (For experimentation only)

[gsvgit]

Looks good.

1. Can you provide an example of end-user code? How caching can be enabled/disabled? On which step it should be done?
2. Is this implementation allows one to enable/disable caching for each specific kernel independently?

[dpanfilyonok]

Usage:

opencl {

}
|> OpenCLEvaluationContext(IsCachingEnabled = true).RunSync

let ctx = OpenCLEvaluationContext()

// without caching
opencl {

}
|>  ctx.RunSync

// with caching
ctx.IsCachingEnabled  <- true
opencl {

}
|>  ctx.RunSync

opencl {
  let kernel1 = <@ .. @>
  let kernel2 = <@ .. @>

  // with caching
  do! RunCommand kernel1 ... 

  // without caching 
  let! ctx = getEvaluationContext
  let oldContext = ctx.Clone()
  ctx.IsCachingEnabled  <- false
  do! RunCommand kernel2

  ctx.IsCachingEnabled  <- oldContext.IsCachingEnabled
}
|> OpenCLEvaluationContext(IsCachingEnabled = true).RunSync

It should be noted that the proposed solution is suitable for experimentation only. It will crash on caching equal expressions but with different types in it (because string representation of Expr erase types).

[simpletonDL]

This solution is suitable for experiments and can also be improved in the future (correct hashing, refactoring). So I think I can make a release

[gsvgit]

@simpletonDL well. Please, publish new package.