C ergonomics

[treeform]

Summary

• Use opaque pointer typedefs for generated C reference handles instead of integer aliases.
• Emit C99 fixed-width integer types for sized Nim integers, intptr_t/uintptr_t for Nim int/uint, and int32_t for C runes.
• Emit const char* for generated C string and cstring types.
• Wrap generated C function declarations in extern C guards for C++ consumers.

Validation

• nim c --mm:arc --app:lib -d:gennyC --nimcache:tests/generated/nimcache_c_ergonomics -o:tests/generated/test_c.dll tests\test.nim
• gcc -Wall -Wextra -pedantic -Werror -o tests/generated/test_c.exe tests/test_c.c -I tests/generated C:/p/genny/tests/generated/test_c.dll
• ./tests/generated/test_c.exe
• g++ -Wall -Wextra -pedantic -Werror -x c++ -std=c++11 -c tests/test_c.c -I tests/generated -o tests/generated/test_c_header_cpp.o
• nim c --mm:arc --app:lib -d:gennyC --path:../genny/src --path:src -o:bindings/generated/pixie.dll bindings/bindings.nim
• gcc -Wall -Wextra -pedantic -o tests/generated/test_pixie_c.exe tests/test_pixie_c.c -I ../pixie/bindings/generated C:/p/pixie/bindings/generated/pixie.dll -lm
• ./tests/generated/test_pixie_c.exe
• g++ -Wall -Wextra -pedantic -Werror -x c++ -std=c++11 -c tests/test_pixie_c.c -I ../pixie/bindings/generated -o tests/generated/test_pixie_c_header_cpp.o