* @panic generates an error return trace
* printing an error return trace no longer interferes with
normal stack traces.
* instead of ignore_frame_count, we look at the return address
when you call panic, and that's the first stack trace function
makes stack traces much cleaner - the error return trace
flows gracefully into the stack trace
The purpose of this is:
* Only one way to do things
* Changing a function with void return type to return a possible
error becomes a 1 character change, subtly encouraging
people to use errors.
See #632
Here are some imperfect sed commands for performing this update:
remove arrow:
```
sed -i 's/\(\bfn\b.*\)-> /\1/g' $(find . -name "*.zig")
```
add void:
```
sed -i 's/\(\bfn\b.*\))\s*{/\1) void {/g' $(find ../ -name "*.zig")
```
Some cleanup may be necessary, but this should do the bulk of the work.
* 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
