Add base statement parsing

Assign statements and standalone expression statements are added, plus
some tests.

Additionally, starting tokens are implemented using lazy_static!{} so
that they can be mixed and matched.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/ast.rs

@@ -95,6 +95,7 @@ pub enum BaseExprKind {
     List(Vec<Expr>),
     Object(Vec<(Expr, Expr)>),
     Tuple(Vec<Expr>),
+    Block(Vec<Stmt>),
 }
 
 #[derive(Derivative, Clone, PartialEq, Eq)]