* docgen supports obj_err code kind for demonstrating
errors without explicit test cases
* add documentation for `extern enum`. See #367
* remove coldcc keyword and add @setIsCold. See #661
* add compile errors for non-extern struct, enum, unions
in function signatures
* add .h file generation for extern struct, enum, unions
* introduce zigrt file. it contains only weak symbols so that
multiple instances can be merged. it contains __zig_panic
so that multiple .o files can call the same panic function.
* remove `@setFnVisible` builtin and add @setGlobalLinkage builtin
which is more powerful
* add `@panic` builtin function.
* fix collision of symbols with extern prototypes and internal
function names
* add stack protector safety when linking against libc. To add
the safety mechanism without libc requires implementing
Thread Local Storage. See #276
Andrew Kelley
Andrea Orru