Add cast and isinstance keywords (#1505)

Finishing support for the type syntax proposed in
https://github.com/bazelbuild/starlark/pull/362

After some reflection, I think we should always parse cast/isinstance as
keywords (requiring 2 arguments, one of which is a value expression and
the other a type expression) since permitting pre-type Starlark syntax
is likely to only lead to confusion, and would be quite difficult to get
right.

Note that this is an **incompatible change**: forms like `cast(x, y, z)`
or `cast = foo` will now be a parse error. However, in practice, I
observe no occurrences of `cast` or `isinstance` used as identifiers in
any Starlark code in Google's monorepo, so I believe that it's rather
unlikely to break anyone.

Store the parsed cast/isinstance exprs as `CallExpr`-s (to avoid
diverging their formatting/walking/etc. support from call expressions)
with an enum field to mark that they are of a special kind.

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