repl: Always use parser, allow trailing ; in assignments

We now properly shell out to the parser instead of hacking stuff
together with a regex. Stuff we get for free by doing this:

- Optional trailing semicolon
- Declaring nested attribute sets
- String identifiers, and future proofing for eventual grammar
improvements to identifiers
- Dynamic attributes

Change-Id: Ibf1ad815e5e27caf162df05ea5ba5b1b4955d9c9

tests/functional/repl_characterization/data/repl_input.test

@@ -0,0 +1,50 @@
+@args -v
+Adding variables gives simple user feedback
+
+  Lix $VERSION
+  Type :? for help.
+  nix-repl> foo = 5
+  Added foo.
+
+  nix-repl> foo = 10
+  Updated foo.
+
+Optional semicolon at the end, allow setting multiple variables in one line
+
+  nix-repl> foo = 2;
+  Updated foo.
+
+  nix-repl> foo = 2; bar = 3;
+  Updated foo.
+  Added bar.
+
+String identifiers work
+
+  nix-repl> "silly name" = null
+  Added "silly name".
+
+Attrset syntax works, but without dynamic attrs or merging
+
+  nix-repl> foo.bar = "baz"
+  Updated foo.
+
+  nix-repl> foo
+  { bar = "baz"; }
+
+  nix-repl> foo."this works" = 42
+  Updated foo.
+
+  nix-repl> foo
+  { "this works" = 42; }
+
+  nix-repl> foo.bar = "baz"; foo.more = "error"
+  error: attribute 'foo' already defined at «string»:1:18
+         at «string»:1:12:
+              1| foo.bar = "baz"; foo.more = "error"
+               |            ^
+
+  nix-repl> ${foo} = 10
+  error: dynamic attributes not allowed in REPL
+         at «string»:1:1:
+              1| ${foo} = 10
+               | ^