Implement slicing of arrays and strings.

Also add a test for slicing strings with utf-8 encoded characters.  Only
bash passes this test.

Also, adjust allowed spec test failures.

Motivation: Aboriginal Linux uses slicing.

core/word_eval.py

@@ -815,10 +815,29 @@ def _EvalBracedVarSub(self, part, quoted):
          raise AssertionError(val.__class__.__name__)

      elif op.tag == suffix_op_e.Slice:
        # Either string slicing or array slicing.  However string slicing has
        # a unicode problem? 
        # Or maybe have a different operator for byte slice and char slice.
        raise NotImplementedError(op)
        # NOTE: The beginning can be negative, but Python handles this.  Might
        # want to make it explicit.
        # TODO: Check out of bounds errors?  begin > end?
        if op.begin:
          begin = self.arith_ev.Eval(op.begin)
        else:
          begin = 0

        if op.length:
          length = self.arith_ev.Eval(op.length)
          end = begin + length
        else:
          end = None  # Python supports None as the end

        if val.tag == value_e.Str:  # Slice characters in a string.
          # TODO: Need to support unicode?  Write spec # tests.
          val = runtime.Str(val.s[begin : end])

        elif val.tag == value_e.StrArray:  # Slice array entries.
          val = runtime.StrArray(val.strs[begin : end])

        else:
          raise AssertionError(val.__class__.__name__)

    # After applying suffixes, process decay_array here.
    if decay_array:

spec/var-op-other.test.sh

@@ -1,6 +1,8 @@
#!/usr/bin/env bash
#
# Test combination of var ops.
#
# NOTE: There are also slice tests in {array,arith-context}.test.sh.

### String length
v=foo
@@ -22,6 +24,7 @@ echo ${#undef}
v=abcde
echo ${#v:1:3}
# status: 1
# OK osh status: 2
# N-I dash status: 0
# N-I dash stdout: 5

@@ -128,3 +131,13 @@ echo ${foo: i-3-2 : i + 2}
# stdout: def
# N-I dash status: 2
# N-I dash stdout-json: ""

### Slice String with Unicode
# mksh slices by bytes.
foo='--μ--'
echo ${foo:1:3}
# stdout: -μ-
# BUG mksh stdout: -μ
# N-I dash status: 2
# N-I dash stdout-json: ""

test/spec.sh

@@ -380,7 +380,7 @@ var-op-test() {
}

var-op-other() {
  sh-spec spec/var-op-other.test.sh --osh-failures-allowed 5 \
  sh-spec spec/var-op-other.test.sh --osh-failures-allowed 2 \
    ${REF_SHELLS[@]} $OSH "$@"
}

@@ -422,12 +422,12 @@ errexit() {

# There as many non-POSIX arithmetic contexts.
arith-context() {
  sh-spec spec/arith-context.test.sh --osh-failures-allowed 7 \
  sh-spec spec/arith-context.test.sh --osh-failures-allowed 3 \
    $BASH $MKSH $ZSH $OSH "$@"
}

array() {
  sh-spec spec/array.test.sh --osh-failures-allowed 10 \
  sh-spec spec/array.test.sh --osh-failures-allowed 7 \
    $BASH $MKSH $OSH "$@"
}