zig build system supports building a library

See #329

Supporting work:
 * move std.cstr.Buffer0 to std.buffer.Buffer
 * add build.zig to example/shared_library/ and add an automated test
   for it
 * add std.list.List.resizeDown
 * improve std.os.makePath
   - no longer recursive
   - takes into account . and ..
 * add std.os.path.isAbsolute
 * add std.os.path.resolve
 * reimplement std.os.path.dirname
   - no longer requires an allocator
   - handles edge cases correctly

src/codegen.cpp

@@ -155,6 +155,12 @@ void codegen_set_test_name_prefix(CodeGen *g, Buf *prefix) {
     g->test_name_prefix = prefix;
 }
 
+void codegen_set_lib_version(CodeGen *g, size_t major, size_t minor, size_t patch) {
+    g->version_major = major;
+    g->version_minor = minor;
+    g->version_patch = patch;
+}
+
 void codegen_set_is_test(CodeGen *g, bool is_test_build) {
     g->is_test_build = is_test_build;
 }