Function growingOStream

growingOStream(initialCapacity?: number): OStream
An OStream whose buffer follows the message — the ready-made form of the caller CORELIB_PLAN §5.1.2 puts the allocation in.

It is the one-liner for the 90% case, where the message comfortably fits in memory:
```
const os = growingOStream();
os.writeUnsigned(1, 42);
const wire = os.bytes();   // the whole message, as a view valid until the next write
```
OStream.bytes is therefore the whole message here rather than a not-yet-flushed tail, and no write reports BUFFER_FULL. It is an ordinary streaming stream in every other respect: OStream.setBuffer works and means what it always means — the not-yet-flushed bytes in the old buffer are dropped and encoding continues into yours, which the accumulator then grows in turn.
Parameters
- initialCapacity: number = DEFAULT_CAPACITY
  bytes to start from; the accumulator enlarges itself whenever the message outgrows it, so this only trades an initial allocation against the number of enlargements and never limits the message. A caller with a rough size in hand should pass it: 100 KB of small fields grows by doubling and costs nine enlargements from the 256-byte default. A single large field does not — a bulk write hands the sink its needed, so the buffer reaches that size in one step and the write keeps its bulk route.
Returns OStream
- Defined in src/encode/accumulate.ts:191