Fix double expansions (`$$foo`)

Double expansions of variables had the following issues:

* `"$$foo"` threw an error no matter what the value of `$foo` was.
* `set -l foo ''; echo $$foo` threw an error because of the expansion of
  `$foo` to `''`.

With this change, double expansion always works properly. When
double-expanding a multi-valued variable, in a double-quoted string the
first word of the inner expansion is used for the outer expansion, and
outside of a quoted string every word is used for the double-expansion
in each of the arguments.

    > set -l foo bar baz
    > set -l bar one two
    > set -l baz three four
    > echo "$$foo"
    one two baz
    > echo $$foo
    one two three four

1 files changed, 64 insertions, 0 deletions

diff --git a/tests/expansion.in b/tests/expansion.in
new file mode 100644
index 00000000..d362e789
--- /dev/null
+++ b/tests/expansion.in
@@ -0,0 +1,64 @@
+# Test expansion of variables
+
+function show --description 'Prints argument count followed by arguments'
+    echo (count $argv) $argv
+end
+
+set -l foo
+show "$foo"
+show $foo
+show "prefix$foo"
+show prefix$foo
+
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l foo ''
+show "$foo"
+show $foo
+show "prefix$foo"
+show prefix$foo
+
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l foo bar
+set -l bar
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l bar baz
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l bar baz quux
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l foo bar fooer fooest
+set -l fooer
+set -l fooest
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo
+
+set -l fooer ''
+show $$foo
+show prefix$$foo
+
+set -l foo bar '' fooest
+show "$$foo"
+show $$foo
+show "prefix$$foo"
+show prefix$$foo