Refactor memory wrappers

[maleadt]

Implements #2236: Each piece of memory now keeps track on which stream is working on it, and whether there's outstanding (non-synchronized) changes within that stream. That makes it possible to accurately and automatically synchronize when we derive a pointer from a CuArray (which is the one and only place where we indicate that we will start using the memory).

I did quite a bit of refactoring as part of this PR, most notably removing the Mem submodule. I added plenty deprecations though, so it should be nonbreaking (although I expect issues and hotfixes when we tag this).

---

Closes #1566 by making synchronization unnecessary, i.e., the following now "just works":

a = CUDA.zeros(4096, 4096)
b = CUDA.zeros(4096, 4096)
t = @async begin
    # PREVIOUSLY WRONG: using `a` while it may not be ready
    for x in 1:100
        mul!(b, a, a)
    end
    a .= 1
end
wait(t)
# PREVIOUSLY WRONG: using `a` while it may not be ready
display(Array(a)[end])

@vchuravy: much easier than the Task changes we were brainstorming about :-)

---

Fixes #2323:

julia> using CUDA

julia> CUDA.device!(0)
CuDevice(0): Tesla V100-PCIE-16GB

julia> x = CUDA.ones(2)

2-element CuArray{Float32, 1, CUDA.DeviceMemory}:
 1.0
 1.0

julia> CUDA.device!(1)
CuDevice(1): Tesla V100S-PCIE-32GB

julia> x
2-element CuArray{Float32, 1, CUDA.DeviceMemory}:
 1.0
 1.0

julia> y = x - x;

julia> CUDA.device(x)
CuDevice(0): Tesla V100-PCIE-16GB

julia> CUDA.device(y)
CuDevice(1): Tesla V100S-PCIE-32GB

julia> y
2-element CuArray{Float32, 1, CUDA.DeviceMemory}:
 0.0
 0.0

---

Hopefully fixes #875

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.30%. Comparing base (c5fcd73) to head (5f4711a).
Report is 2 commits behind head on master.

❗ Current head 5f4711a differs from pull request most recent head 92ac27b. Consider uploading reports for the commit 92ac27b to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2335       +/-   ##
===========================================
- Coverage   71.86%   60.30%   -11.57%     
===========================================
  Files         155      155               
  Lines       15020    14880      -140     
===========================================
- Hits        10794     8973     -1821     
- Misses       4226     5907     +1681     

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