first pass at zig build system

* `zig build --export [obj|lib|exe]` changed to `zig build_obj`,
   `zig build_lib` and `zig build_exe` respectively.
 * `--name` parameter is optional when it can be inferred from the
   root source filename. closes #207
 * `zig build` now looks for `build.zig` which interacts with
   `std.build.Builder` to describe the targets, and then the zig
   build system prints TODO: build these targets. See #204
 * add `@bitcast` which is mainly used for pointer reinterpret
   casting and make explicit casting not do pointer reinterpretation.
   Closes #290
 * fix debug info for byval parameters
 * sort command line help options
 * `std.debug.panic` supports format string printing
 * add `std.mem.IncrementingAllocator`
 * fix const ptr to a variable with data changing at runtime.
   closes #289

src/codegen.hpp

@@ -45,6 +45,7 @@ void codegen_set_mios_version_min(CodeGen *g, Buf *mios_version_min);
 void codegen_set_linker_script(CodeGen *g, const char *linker_script);
 void codegen_set_omit_zigrt(CodeGen *g, bool omit_zigrt);
 
+PackageTableEntry *new_package(const char *root_src_dir, const char *root_src_path);
 void codegen_add_root_code(CodeGen *g, Buf *source_dir, Buf *source_basename, Buf *source_code);
 
 void codegen_parseh(CodeGen *g, Buf *src_dirname, Buf *src_basename, Buf *source_code);