Implement module-level memoization of inlined requires (#1346)

Summary:
Pull Request resolved: #1346

A performance optimisation for `inlineRequries`, where we replace `const foo = require('./foo')` at top level with `var foo`, and replace subsequent references to `foo` with `(foo || foo = require('./foo')`.

This differs from plain `inlineRequires` by the use of the memo variable, which (for modules that don't return a falsy value) saves calling into `require`, backed by `metroRequire`, and retrieving the previously-initialised module from a `Map` by its numeric ID.

```
 - **[Experimental]:** Implement `transformer.unstable_memoizeInlineRequires` to optimise inlined access.
```

Reviewed By: javache

Differential Revision: D62125134

fbshipit-source-id: 1ddbd8051d4a00b0ec358be8838f43eb8b0144a0

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -152,6 +152,7 @@ Object {
     "unstable_dependencyMapReservedName": null,
     "unstable_disableModuleWrapping": false,
     "unstable_disableNormalizePseudoGlobals": false,
+    "unstable_memoizeInlineRequires": false,
     "unstable_renameRequire": true,
     "unstable_workerThreads": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
@@ -334,6 +335,7 @@ Object {
     "unstable_dependencyMapReservedName": null,
     "unstable_disableModuleWrapping": false,
     "unstable_disableNormalizePseudoGlobals": false,
+    "unstable_memoizeInlineRequires": false,
     "unstable_renameRequire": true,
     "unstable_workerThreads": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
@@ -516,6 +518,7 @@ Object {
     "unstable_dependencyMapReservedName": null,
     "unstable_disableModuleWrapping": false,
     "unstable_disableNormalizePseudoGlobals": false,
+    "unstable_memoizeInlineRequires": false,
     "unstable_renameRequire": true,
     "unstable_workerThreads": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
@@ -698,6 +701,7 @@ Object {
     "unstable_dependencyMapReservedName": null,
     "unstable_disableModuleWrapping": false,
     "unstable_disableNormalizePseudoGlobals": false,
+    "unstable_memoizeInlineRequires": false,
     "unstable_renameRequire": true,
     "unstable_workerThreads": false,
     "workerPath": "metro/src/DeltaBundler/Worker",

packages/metro-config/src/defaults/index.js

@@ -137,6 +137,7 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
     unstable_disableNormalizePseudoGlobals: false,
     unstable_renameRequire: true,
     unstable_compactOutput: false,
+    unstable_memoizeInlineRequires: false,
     unstable_workerThreads: false,
   },
   watcher: {