License is missing on some includes

[AndWass]

Currently there is no licensing information for algorithm_ext.h ring.h and transcode.h. This should probably be added if the code should be used by others.

[Quuxplusone]

I'd say we should add the Boost license to those files as well. Contributors ought to sign off on the idea, though. Can the following contributors please 👍 or 👎 the idea of Boost-licensing all the files to which they've made commits, listed below?

ring.h: @hatcat @SuperV1234 @davidziman @Quuxplusone

algorithm_ext.h: @briterator @Quuxplusone and one commit by <john@john-debian> whoever that is

transcode.h: @hatcat @p-groarke