String conversions

[Rantanen]

Expose all strings as BSTR for now over COM. Handle conversions between BSTR and inner string types in a general way using FromWithTemporary trait.

• [x] Implement BString/BStr duality to allow copy-free BSTR parameters.
• [x] Implement traits for defining various string conversions that might or might not need temporary variables.
• [x] Implement parameter validation in Intercom-internals in places where the COM type is something like BSTR that may contain data that is not compatible with the Rust type that might be String for example.
• [x] Implement intercom::CString/CStr duality to allow copy-free char* parameters.
• [x] Update test output
• [x] Write tests for passthrough string APIs that call various BSTR/char* methods with parameters and do conversions.
• [x] Write tests for BSTR conversions to ensure null/normal/multibyte UTF-8/UTF-16 strings go through both ways as expected.
• [x] Write tests for BSTR string types as parameters (&BStr, BString) and ensure these are unallocated.
• [x] Re-introduce the utf8 conversion tests in cpp-wrapper tests.
• [x] Ensure tests run in Unix
• [x] Finalize documentation, refine public trait/etc. names.

I'll split the CString/CStr and interface selection into a different PR.

[codecov-io]

Codecov Report

Merging #65 into master will decrease coverage by 1.02%. The diff coverage is 75.58%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   74.04%   73.01%   -1.03%     
==========================================
  Files          56       46      -10     
  Lines        3467     3194     -273     
==========================================
- Hits         2567     2332     -235     
+ Misses        900      862      -38

Impacted Files	Coverage Δ
intercom-common/src/builtin_model.rs	91.3% <ø> (ø)	:arrow_up:
intercom-common/src/generators/cpp.rs	80.58% <100%> (+0.07%)	:arrow_up:
intercom/src/alloc.rs	70% <100%> (+67.36%)	:arrow_up:
intercom-common/src/generators/idl.rs	82.11% <100%> (ø)	:arrow_up:
intercom-common/src/attributes/com_interface.rs	98.68% <100%> (-0.02%)	:arrow_down:
intercom-common/src/attributes/com_impl.rs	100% <100%> (+1.75%)	:arrow_up:
intercom/src/strings.rs	67.19% <67.19%> (ø)
intercom-common/src/methodinfo.rs	84.81% <70.37%> (-1.55%)	:arrow_down:
intercom-common/src/tyhandlers.rs	83.49% <83.87%> (+10.63%)	:arrow_up:
intercom-common/src/returnhandlers.rs	86.91% <89.18%> (-1.46%)	:arrow_down:
... and 13 more

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[Rantanen]

DONE! \o/

Functionality wise anyway. The remaining work is turning ComResult into a result of ComError instead of HRESULT and then implementing the Utf8String. These will happen in different PRs.