zig build system improvements, add some std API

* add std.buf_map.BufMap
 * add std.buf_set.BufSet
 * add std.mem.split
 * zig build system improvements (See #204)
   - automatically parses NIX_CFLAGS_COMPILE and NIX_LDFLAGS
   - add builder.addCIncludePath
   - add builder.addRPath
   - add builder.addLibPath
   - add exe.linkLibrary

std/special/build_runner.zig

@@ -20,6 +20,7 @@ pub fn main() -> %void {
     defer inc_allocator.deinit();
 
     var builder = Builder.init(zig_exe, &inc_allocator.allocator);
+    defer builder.deinit();
     root.build(&builder);
     %return builder.make(leftover_arg_index);
 }