Add an option to alias global objects

This for example allows to switch the default 'Math' implementation a program will use. Uses 'NativeMath' by default.

src/compiler.ts

@@ -155,6 +155,8 @@ export class Options {
   memoryBase: u32 = 0;
   /** If true, generates information necessary for source maps. */
   sourceMap: bool = false;
+  /** Global aliases. */
+  globalAliases: Map<string,string> | null = null;
 
   /** Tests if the target is WASM64 or, otherwise, WASM32. */
   get isWasm64(): bool {