Add type parameter to Kernel::send specifying type for receiving kernel

[anorth]

This type parameter supports a kernel to be wrapped, and have the wrapped type also be instantiated on the other side of the send. Something like this is necessary to support injecting kernel behaviour for testing.

@alexytsu and I couldn't get the TestKernel/TestCallManager pattern to work from github.com/anorth/fvm-workbench (where we need to fake out proof verification, and long term wish to be able to inject arbitrary kernel behaviour). This new scheme avoids the coupled TestCallManager and associated funky transmutation.

It's limited in that, unless we can figure out the right type constraints, a TestCallManager itself can't be wrapped, because it ignores the type parameter provided to it.

This is a partial resolution of #1779 (we're unblocked for now). But as noted there, still very limited because there's no facility for dynamic behaviour of an injected kernel. The kernel is constructed internally and short-lived. Passing a kernel construction method (i.e. factory) at runtime would be far more flexible.

[codecov-commenter]

Codecov Report

Merging #1780 (ae371e6) into master (f4f3f34) will increase coverage by 0.01%.
The diff coverage is 98.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1780      +/-   ##
==========================================
+ Coverage   74.75%   74.76%   +0.01%     
==========================================
  Files         146      146              
  Lines       14106    14106              
==========================================
+ Hits        10545    10547       +2     
+ Misses       3561     3559       -2     

Impacted Files	Coverage Δ
fvm/src/kernel/default.rs	53.10% <97.91%> (ø)
fvm/src/syscalls/send.rs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[Stebalien]

I wish there were a better way... but this definitely makes testing easier.