Upgrade to use Irmin 2.6 API

[craigfe]

Changes are as follows:

• provide clear operations for the raw stores (via a binding to IDBObjectStore.clear);
• fix usages of Irmin.Type generic functions to be properly staged.

[talex5]

Thanks!

Should the clear function also be tested in https://github.com/talex5/irmin-indexeddb/blob/master/test/test.ml, or does it get tested automatically via some other route? Note that you have to open test.html in a browser to run the tests.

(also, CI is failing)

[craigfe]

I'll add a quick test for clear as you suggest.

Once this is included in the Irmin repository, we have some plumbing to subject each backend to a standard set of tests that already includes clear.

[talex5]

The tests fail for me with:

Created Git-format basic store. Checking it is empty...
Got [], as expected
ERROR: File "src/irmin/tree.ml", line 1279, characters 6-12: Assertion failed

Also, we should probably disable Travis.