Breaking: upgrade to abstract-level 2.0.0

[vweevers]

One of the abstract-level tests fails (data comes out as a buffer or view but abstract-level expects a string), need to check why.

prewrite hook function can write to nondescendant sublevel
    ✖ 8059) should be deeply equivalent
      @@ -1,1 +1,1 @@
      -"[ '!books!12', '!authors!\"Hesse~12\"' ]"
      +"[ Uint8Array [Uint8Array] { 0: 33, 1: 98, 2: 111, 3: 111, 4: 107, 5: 115, 6: 33, 7: 49, 8: 50 }, Uint8Array [Uint8Array] { 0: 33, 1: 97, 2: 117, 3: 116, 4: 104, 5: 111, 6: 114, 7: 115, 8: 33, 9: 34, 10: 72, 11: 101, 12: 115, 13: 115, 14: 101, 15: 126, 16: 49, 17: 50, 18: 34 } ]"
      At: AbstractSublevel.<anonymous> (\node_modules\abstract-level\test\hooks\prewrite.js:782:9)

Other than that this is a straight forward change.