Migrating opus-codec.org to a static site generator inserted
a redirect into the download path, which the actual testvectors
at https://opus-codec.org/static/testvectors/opus_testvectors.tar.gz.

Add the -L option to the suggested curl invocation so the redirect
is followed to download the actual resource. Without this one
gets the html redirection fallback page.

Makes things a little easier to read.

Signed-off-by: Gian-Carlo Pascutto <gcp@sjeng.org>

Make it easier for users unfamiliar with C applications
to installed the necessary build dependencies.

Signed-off-by: Gian-Carlo Pascutto <gcp@sjeng.org>

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

Also fixes a minor typo.

Renames test_opus to opus_demo and adds the test_opus_api, test_opus_encode, test_opus_decode test programs.

Updated README files to reflect current output of tool and tweaked punctuation and wording. Corrected typo in test_opus ("constraint"->"constrained").

Also a CELT update