parser_util: add `interactive_parse` to have a simple AST from the user

Signed-off-by: Jean Privat <jean@pryen.org>
test_parser: add option `-i` for interactive parsing

Signed-off-by: Jean Privat <jean@pryen.org>
tests: tests -i on test_parser

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: Interactive parser

Just to play with `parse_something`

~~~
$ test_parser -i -n
--> hello world
Got ABlockExpr
--> "hello world"
Got AStringExpr
--> for x in [1,2,3] do
... print x
... end
Got ABlockExpr
--> fun foo do
... print "hello"
... end
Got AModule
--> for x in in in p
        for x in in in p
                 ^: Syntax error: unexpected keyword 'in'.
--> :q
~~~

Pull-Request: #424
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>