Specify that the size of an array must be a local compile-time known value

[shyukahn]

The spec currently states that the size of an array is a compile-time known value, but not local. On the other hand, the size of a header stack must be a local compile-time known value, according to the spec. This PR revises the spec to specify that the size of array must also be a local compile-time known value.

For reference, p4c disallows arrays with a non-local compile-time known value, e.g.

control proto();
package top(proto p);

control c()(bit<8> n) {
    apply {
        bit<32>[n] arr;
    }
}

top(c(2)) main;

yields an error:

array.p4(6): [--Werror=type-error] error: bit<32>[?]: Size of header stack type should be a constant
        bit<32>[n] arr;
        ^^^^^^^^^^