encode.CHARSTRING is often used on the exact same objects
-> dirty perf optim: memoize the result using a weakmap
Remove WeakMap polyfill, only use it when native  implementation is available
And don't require the polyfill if you don't use it!
Merge branch 'perfOptim' of https://github.com/byte-foundry/opentype.js into byte-foundry-perfOptim
Merge branch 'byte-foundry-perfOptim'