WTF-8 strings, big BigInt, and the honest benchmark pack

Strings gain real lone-surrogate support, BigInt gains a two-tier i128 plus num-bigint implementation, and bench/real arrives as a ten-workload five-engine suite.

c43b17ce: WTF-8 so that a lone surrogate survives a round trip, as ECMAScript requires. 7f4e58b6: BigInt with an i128 fast tier backed by num-bigint. f00c2010: "Add bench/real: ten-workload five-engine suite (the honest pack)", the ancestor of today's real13 suite. The day before, b2c04ace had given Atomics a real blocking wait with a global FIFO waiter registry and cross-thread notify, and 247238ca had added proper tail calls.