irmin: drop phantom 'b from Heap.t, consolidate test suite

Heap.t: ('h, 'v, 'b) t → ('h, 'v) t. The phantom 'b parameter
served no purpose — layer, recording, and of_list produce heaps
with no meaningful backend tag. The record-of-closures is the
correct representation for this case (composable heaps can't be
expressed as a single BACKEND module).

Tests: move schema and tar tests from irmin/test/{schema,tar}/
into irmin/test/test_{schema,tar}.ml. All 19 tests now run from
a single `dune exec irmin/test/test.exe`.