Implement basic bar ref ${!a}.

Polish var-ref tests.

Addresses issue #26.

core/word_eval.py

@@ -434,8 +434,11 @@ def _ApplyPrefixOp(self, val, op_id):
        # TODO: There can be empty placeholder values in the array.
        length = len(val.strs)
      return runtime.Str(str(length))
    elif op_id == Id.VSub_Bang:
      # Treat the value of the variable as a variable name.
      return self.mem.GetVar(val.s)
    else:
      raise NotImplementedError(op_id)
      raise AssertionError(op_id)

  def _ApplyOtherSuffixOp(self, val, op):

spec/var-ref.test.sh

@@ -4,6 +4,30 @@
#
# http://stackoverflow.com/questions/16461656/bash-how-to-pass-array-as-an-argument-to-a-function

### var ref ${!a}
a=b
b=c
echo ref ${!a} ${a}
# Woah mksh has a completely different behavior -- var name, not var ref.
# stdout: ref c b
# BUG mksh stdout: ref a b
# N-I dash/zsh stdout-json: ""

### declare -n and ${!a}
declare -n a
a=b
b=c
echo ${!a} ${a}
# stdout: b c
# N-I mksh stdout: a b

### Bad var ref with ${!a}
#set -o nounset
a='bad var name'
echo ref ${!a}
echo status=$?
# stdout-json: "ref\nstatus=0\n"
# BUG mksh stdout-json: "ref a\nstatus=0\n"
### pass array by reference
show_value() {
  local -n array=$1
@@ -42,20 +66,3 @@ caller
# BUG mksh stdout-json: ""
# BUG mksh status: 1

### Var ref with ${!a}
a=b
b=c
echo ref ${!a}
# Woah mksh has a completely different behavior -- var name, not var ref.
# stdout: ref c
# BUG mksh stdout: ref a
# N-I dash/zsh stdout-json: ""

### Bad var ref with ${!a}
#set -o nounset
a='bad var name'
echo ref ${!a}
# Woah even dash implements this!
# stdout-json: "ref\n"
# BUG mksh stdout: ref a
# N-I dash/zsh stdout-json: ""

test/spec.sh

@@ -452,7 +452,8 @@ extended-glob() {

# ${!var} syntax -- oil should replace this with associative arrays.
var-ref() {
  sh-spec spec/var-ref.test.sh $BASH $MKSH "$@"
  sh-spec spec/var-ref.test.sh --osh-failures-allowed 4 \
    $BASH $MKSH $OSH "$@"
}

let() {