Hide private instance state

[dgraham]

Move private instance variables into a WeakMap so they aren't exposed as public API on the element. The private variables are removed from the type declaration file as well.

$ diff dist-master/index.d.ts dist/index.d.ts

<     currentQuery: string | null;
<     debounceInputChange: (event: Event) => void;
<     boundFetchResults: (event: Event) => void;