Fix @const annotation

[raynei86]

The following code will not compile:

/**
 * @ensure const(bar)
 **/
fn void foo(int bar) {}

The compiler gives this error:

/**
 * @ensure const(bar)
           ^^^^^
Error: An expression was expected.

And when you change it up to this:

/**
 * @const bar
 **/
fn void foo(int bar) {}

The error becomes:

/**
 * @const bar
          ^^^
Error: Expected end of line.

[lerno]

I think I might remove this annotation. It's not super useful when in/out/inout exists.