Store a sent packet's first stream send inline

[rnro]

Every packet that carries stream data records which stream sends it carried in
TransmittedItems.sentStreams, and keeps the record until the packet is
acknowledged or declared lost. The record was an array, so each such packet
allocated storage for it, though a packet almost always carries one stream.
SentStreams keeps the first send inline and only allocates for the rest.
SentStream holds nothing but values, so it is now copyable, which lets the
inline slot be an ordinary optional.

Measured on my Mac against main with the package's QUIC benchmark tools.
Allocation counts come from full malloc stack logging, which records every
allocation:

QUICTransfer -size 1200, per message      24.37 -> 23.31
QUICTransfer, per 500 KB transfer       6,187.6 -> 5,747.9
QUICHandshake, per connection           1,947.4 -> 1,944.8
QUICStreamLoad, per stream                111.8 -> 109.8

The stream load still allocates for a second send in about one packet in
seventeen.

Wall-clock time comes from running main, this change and the other changes
measured alongside it in a rotating order for 9 rounds, and comparing each run
with main's in the same round. Changes moved paths they do not touch by up to
about 1.3%, so differences that size count as noise. Nothing changed beyond
noise.

[tfpauly]

Seems like this could use the UniqueSmallArray from here: https://github.com/apple/swift-network-evolution/pull/131/changes#diff-e7c6fbc146fbc07403b5f53246149d02bb76bdd57e3ee7f871263abd3206e9cf

Which was waiting on swiftlang/swift#91664