[RFC] Allow to cast an any pointer to a dynamic value

[marmeladema]

This is just meant to start the discussion for #565, not be a full implementation yet.

The idea is to allow one to cast an any pointer to a dynamic value.
But in order to do that, the caller must provide the type of the value.
The part I am not so sure about is whether the function is safe to call, even if the type is wrong.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 50.64%. Comparing base (ab342b3) to head (ae11091).
Report is 157 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/any_pointer.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
- Coverage   51.64%   50.64%   -1.00%     
==========================================
  Files          69       70       +1     
  Lines       33735    32239    -1496     
==========================================
- Hits        17422    16328    -1094     
+ Misses      16313    15911     -402     

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