bridge: Add back support for bridging u64, now losslessly

[jrose-signal]

• FFI: still maps trivially to uint64_t, no change
• JNI: reinterpret as jlong (i64) to not lose any information
• Node: until Neon supports bigints, convert to/from a big-endian Buffer

Follow-up to #404.

[jrose-signal]

What is the plan for node bigint support?

If we decide it's important I could spend some time doing the necessary work on the Neon side; if not, converting from Buffer to bigint on the TypeScript side isn't bad, and converting from bigint to Buffer…probably deserves a helper function. Here's what it looks like in the TS zkgroup port:

if (receiptExpirationTime < 0 || receiptExpirationTime > UINT64_MAX) {
  throw new RangeError(`invalid receipt expiration time ${receiptExpirationTime}`);
}
const receiptExpirationAsBuffer = Buffer.alloc(8);
receiptExpirationAsBuffer.writeBigUInt64BE(receiptExpirationTime);