breaking changes to zig build API and improved caching

* in Zig build scripts, getOutputPath() is no longer a valid function
   to call, unless setOutputDir() was used, or within a custom make()
   function. Instead there is more convenient API to use which takes
   advantage of the caching system. Search this commit diff for
   `exe.run()` for an example.
 * Zig build by default enables caching. All build artifacts will go
   into zig-cache. If you want to access build artifacts in a convenient
   location, it is recommended to add an `install` step. Otherwise
   you can use the `run()` API mentioned above to execute programs
   directly from their location in the cache. Closes #330.
   `addSystemCommand` is available for programs not built with Zig
   build.
 * Please note that Zig does no cache evicting yet. You may have to
   manually delete zig-cache directories periodically to keep disk
   usage down. It's planned for this to be a simple Least Recently
   Used eviction system eventually.
 * `--output`, `--output-lib`, and `--output-h` are removed. Instead,
   use `--output-dir` which defaults to the current working directory.
   Or take advantage of `--cache on`, which will print the main output
   path to stdout, and the other artifacts will be in the same directory
   with predictable file names. `--disable-gen-h` is available when
   one wants to prevent .h file generation.
 * `@cImport` is always independently cached now. Closes #2015.
   It always writes the generated Zig code to disk which makes debug
   info and compile errors better. No more "TODO: remember C source
   location to display here"
 * Fix .d file parsing. (Fixes the MacOS CI failure)
 * Zig no longer creates "temporary files" other than inside a
   zig-cache directory.

This breaks the CLI API that Godbolt uses. The suggested new invocation
can be found in this commit diff, in the changes to `test/cli.zig`.

src/ast_render.cpp

@@ -470,6 +470,9 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                         fprintf(ar->f, ", ");
                     }
                 }
+                if (node->data.fn_proto.is_var_args) {
+                    fprintf(ar->f, ", ...");
+                }
                 fprintf(ar->f, ")");
                 if (node->data.fn_proto.align_expr) {
                     fprintf(ar->f, " align(");