Fix build on previous versions of rust

[TheBlueMatt]

Switching to a function due to deprecation when it isn't even deprecated yet, let alone removed, seems significantly premature, and breaks lots of downstream users.

Addresses the non-meta part of #364.

[alexcrichton]

Switching to a function due to deprecation when it isn't even deprecated yet

This is not true, the deprecation is riding the trains from nightly to stable. The rust standard library never removes stable functions, and users always have the option of sticking with the old functions if desired.

seems significantly premature

This crate's policy on version requirements is the current and prior stable, which makes a window of at least 6 weeks if not more for including new features.

breaks lots of downstream users

I'm sorry for the breakage, it's never fun! I would recommend a Cargo.lock file to prevent unwanted upgrades, as this crate is not the only one which has the same version policy.

[TheBlueMatt]

See my #364 (comment) comment on the associated issue, but, indeed, I'd missed that deny(warnings) was preventing compilation on nightly/CI instead of just generating a warning as I expected. In any case, I laid out the reasons why such breakage is not resolved with cargo vendor there. Sadly the https://github.com/rust-bitcoin/ projects are slowly being forced to cut out every dependency that isn't another project maintained by the same people to maintain stability and satisfy customers.