Fix memory leak in vector slice assignment

Two parts to this:

  * Only allow vector slice assignment in statement contexts, not in
    arbitrary assignment expressions.  E.g. it's not clear what the
    resulting value of `(v[1:2] = vector(1))` is for further expression
    chaining.  For reference, Python doesn't allow it either.

  * Add a subclass of AssignExpr to specialize the behavior for index
    slice assignments (because its behavior regarding expression
    chaining is different per the previous point) and Unref the RHS
    of things like `v[1:2] = vector(1)` after IndexExpr::Assign is
    finished inserting it (since no one else takes ownership of it).

Instead of using an Expr subclass, IndexSliceAssignExpr, we could
use a proper Stmt, since that's the only context we currently use it
for, but if we did ever to decide on allowing its use in arbitrary
expression contexts, then I expect we'll need it this way anyway
(just with a different IndexSliceAssignExpr::Eval implementation).

src/SerialTypes.h

@@ -166,6 +166,7 @@ SERIAL_EXPR(CAST_EXPR, 45)
 SERIAL_EXPR(IS_EXPR_, 46) // Name conflict with internal SER_IS_EXPR constant.
 SERIAL_EXPR(BIT_EXPR, 47)
 SERIAL_EXPR(COMPLEMENT_EXPR, 48)
+SERIAL_EXPR(INDEX_SLICE_ASSIGN_EXPR, 49)
 
 #define SERIAL_STMT(name, val) SERIAL_CONST(name, val, STMT)
 SERIAL_STMT(STMT, 1)