cargo future-incompatibilities warnings for v0.5.0

[jamesyoungman]

There are quite a lot of incompatibility warnings, but only of 2 distinct types:

$ cargo report future-incompatibilities --id 1 --package flac@0.5.0 | grep warning: | sort -u
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
> warning: trailing semicolon in macro used in expression position