Fix errors caused by numeric IDs that are too big for JS numbers

[tjennison-work]

• Numbers in JS are doubles by default. This generally works fine for numeric data but can cause truncation for IDs that use the entire space of an int64.
• The JSON functions can't correctly handle the large numbers without some change to the format. The easiest is to send int64s as strings.
• Those numbers are converted to BigInts to make them numeric but retain their precision. This requires using them throughout the code since our keys/ids are generally int64s.
• BigInts don't follow the same coercion rules as numbers, so many places that previously just worked (e.g. data maps, JSX keys) no longer do. Convert data maps to actual Map objects and explicitly convert keys to strings.
• The change to Maps exposes some clunkiness in the way TreeGrid data is handled. Sticking to a simple conversion for now but will consider other changes in a followup.