webassembly: Add JavaScript proxying, and js and jsffi modules.

This commit improves the webassembly port by adding:

- Proxying of Python objects to JavaScript with a PyProxy type that lives
  on the JavaScript side.  PyProxy implements JavaScript Proxy traps such
  as has, get, set and ownKeys, to make Python objects have functionality
  on the JavaScript side.

- Proxying of JavaScript objects to Python with a JsProxy type that lives
  on the Python side.  JsProxy passes through calls, attributes,
  subscription and iteration from Python to JavaScript.

- A top-level API on the JavaScript side to construct a MicroPython
  interpreter instance via `loadMicroPython()`.  That function returns an
  object that can be used to execute Python code, access the Python globals
  dict, access the Emscripten filesystem, and other things.  This API is
  based on the API provided by Pyodide (https://pyodide.org/).  As part of
  this, the top-level file is changed from `micropython.js` to
  `micropython.mjs`.

- A Python `js` module which can be used to access all JavaScript-side
  symbols, for example the DOM when run within a browser.

- A Python `jsffi` module with various helper functions like
  `create_proxy()` and `to_js()`.

- A dedenting lexer which automatically dedents Python source code if every
  non-empty line in that source starts with a common whitespace prefix.
  This is very helpful when Python source code is indented within a string
  within HTML or JavaScript for formatting reasons.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/main.c

@@ -40,7 +40,9 @@
 #include "shared/runtime/pyexec.h"
 
 #include "emscripten.h"
+#include "lexer_dedent.h"
 #include "library.h"
+#include "proxy_c.h"
 
 #if MICROPY_ENABLE_COMPILER
 int do_str(const char *src, mp_parse_input_kind_t input_kind) {
@@ -113,6 +115,60 @@ void mp_js_init_repl() {
     pyexec_event_repl_init();
 }
 
+void mp_js_register_js_module(const char *name, uint32_t *value) {
+    mp_obj_t module_name = MP_OBJ_NEW_QSTR(qstr_from_str(name));
+    mp_obj_t module = proxy_convert_js_to_mp_obj_cside(value);
+    mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
+    mp_map_lookup(mp_loaded_modules_map, module_name, MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = module;
+}
+
+void mp_js_do_import(const char *name, uint32_t *out) {
+    nlr_buf_t nlr;
+    if (nlr_push(&nlr) == 0) {
+        mp_obj_t ret = mp_import_name(qstr_from_str(name), mp_const_none, MP_OBJ_NEW_SMALL_INT(0));
+        // Return the leaf of the import, eg for "a.b.c" return "c".
+        const char *m = name;
+        const char *n = name;
+        for (;; ++n) {
+            if (*n == '\0' || *n == '.') {
+                if (m != name) {
+                    ret = mp_load_attr(ret, qstr_from_strn(m, n - m));
+                }
+                m = n + 1;
+                if (*n == '\0') {
+                    break;
+                }
+            }
+        }
+        nlr_pop();
+        proxy_convert_mp_to_js_obj_cside(ret, out);
+    } else {
+        // uncaught exception
+        proxy_convert_mp_to_js_exc_cside(nlr.ret_val, out);
+    }
+}
+
+void mp_js_do_exec(const char *src, uint32_t *out) {
+    // Collect at the top-level, where there are no root pointers from stack/registers.
+    gc_collect_start();
+    gc_collect_end();
+
+    mp_parse_input_kind_t input_kind = MP_PARSE_FILE_INPUT;
+    nlr_buf_t nlr;
+    if (nlr_push(&nlr) == 0) {
+        mp_lexer_t *lex = mp_lexer_new_from_str_len_dedent(MP_QSTR__lt_stdin_gt_, src, strlen(src), 0);
+        qstr source_name = lex->source_name;
+        mp_parse_tree_t parse_tree = mp_parse(lex, input_kind);
+        mp_obj_t module_fun = mp_compile(&parse_tree, source_name, false);
+        mp_obj_t ret = mp_call_function_0(module_fun);
+        nlr_pop();
+        proxy_convert_mp_to_js_obj_cside(ret, out);
+    } else {
+        // uncaught exception
+        proxy_convert_mp_to_js_exc_cside(nlr.ret_val, out);
+    }
+}
+
 #if MICROPY_GC_SPLIT_HEAP_AUTO
 
 // The largest new region that is available to become Python heap.