Basic support for @external support, only basic generation and compil…

[jwillemsen]

…er test when using external as struct member, not complete yet

* tests/idl4/external/client.cpp:
* tests/idl4/external/run_test.pl:
* tests/idl4/external/test.idl:
* tests/idl4/external/test.mpc:
  Added.

* ridlbe/c++11/templates/cli/hdr/struct_idl_traits_def.erb:
* ridlbe/c++11/visitors/struct.rb:
* tests/annotations/run_test.pl:
* tests/idl4/bitset/run_test.pl:
* tests/idl4/enum/run_test.pl:
* tests/idl4/explicit_ints/run_test.pl:
* tests/idl4/optional/run_test.pl:
* tests/idl4/union/run_test.pl:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a0b82a7) 59.76% compared to head (d0b3f45) 59.77%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #352   +/-   ##
=======================================
  Coverage   59.76%   59.77%           
=======================================
  Files         588      589    +1     
  Lines       47992    47997    +5     
  Branches     4233     4233           
=======================================
+ Hits        28684    28689    +5     
  Misses      16187    16187           
  Partials     3121     3121           

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