17ca5018eb units: Comment alloc feature (Tobin C. Harding)
Pull request description:
Add a comment to the `serde` module about why we have all the `alloc` feature gating.
Done as part of #3556 - auditing the whole crate for use of `alloc` feature.
ACKs for top commit:
apoelstra:
ACK 17ca5018ebb28d5d392d61aee62170fb3e4f2a5b; successfully ran local tests
Tree-SHA512: 6d13129903a9c530db811e9f64ae4fa653dbcc952a42502c53c6c622a80c1e58b3037edd0638de78867823ca370b3a4595a08a9ec07b84c71afb6200454f5d6d