Implement JS async functions and await #277

Implements issue 3mhn3pphkyd2f

Changes #

Bytecode #

• New Await opcode (0x82) — saves async function state and returns to the driver
• is_async flag on bytecode Function struct

Compiler #

• Async function declarations, expressions, and methods set is_async on compiled function
• await expr compiles to Await opcode
• Async arrow functions set is_async on inner function
• for await (let x of iter) parsed and compiled with @@asyncIterator

VM #

• Async functions create an internal generator + result promise
• drive_async_step() runs the generator and resolves/rejects the promise
• setup_async_resume() wires up promise reactions for await resume
• throw_into_generator() handles rejected await (throws into try/catch)
• Exception handlers saved/restored across yield/await suspension points
• Async generator wrapper with next(), return(), @@asyncIterator
• __async_resume__ detection in both Call opcode and call_function

Tests #

16 unit tests covering:

• Async function returns promise, resolves with return value
• await on resolved/rejected promises
• await on non-promise values
• Multiple sequential awaits
• try/catch around rejected await
• Throw inside async function rejects promise
• Async arrow functions (expression and concise body)
• Await on chained promises
• Async closures
• Async object methods
• Async generators
• for await...of parsing