Support for the lighter ppx syntax: {foo%expect| ... |foo}

[smondet]

Right now (0.14.1) when using {%expect| ...|} if the test succeeds it works, but if it fails the test-driver crashes while trying to make the corrected file:

Fatal error: exception ("Expect test evaluator bug" (exn (Invalid_argument "Negative length: -8"))
  (backtrace
     "Raised at Base__Exn.protectx in file \"src/exn.ml\", line 71, characters 4-114\
    \nCalled from Ppx_expect_evaluator.process_group.write_corrected in file \"evaluator/ppx_expect_evaluator.ml\" (inlined), line 181, characters 6-88\
    \nCalled from Ppx_expect_evaluator.process_group in file \"evaluator/ppx_expect_evaluator.ml\", line 191, characters 10-45\
    \nCalled from Ppx_expect_evaluator.evaluate_tests.(fun) in file \"evaluator/ppx_expect_evaluator.ml\", line 218, characters 6-83\
    \n")
  (filename lib/stdmisc/v0.ml))
Raised at Base__Error.raise in file "src/error.ml" (inlined), line 8, characters 14-30
Called from Base__Error.raise_s in file "src/error.ml", line 9, characters 19-40
Called from Base__List.count_map in file "src/list.ml", line 387, characters 13-17
Called from Base__List.map in file "src/list.ml" (inlined), line 418, characters 15-31
Called from Ppx_expect_evaluator.evaluate_tests in file "evaluator/ppx_expect_evaluator.ml" (inlined), line 216, characters 5-486
Called from Ppx_expect_evaluator.evaluate_tests in file "evaluator/ppx_expect_evaluator.ml", line 215, characters 2-572
Called from Stdlib__List.map in file "list.ml", line 92, characters 20-23
Called from Stdlib__List.map in file "list.ml", line 92, characters 32-39
Called from Ppx_inline_test_lib__Runtime.exit in file "runtime-lib/runtime.ml", line 615, characters 2-49