EME migrated to Contiguous IDL

I merged this PR: https://github.com/w3c/encrypted-media/pull/274

So, I recommend merging from the main branch into any work you have open
sooner rather than later, as the extensive moving around may result in
conflicts.

Also, please check we haven't lost anything important in this migration.

I have a suspicion there is a problem with links to "mediaKeys" and
"MediaKeys" since the contiguous IDL processing seems to generate fragment
names only in all lower case ...

...Mark