Return the validated expression when possible.

By returning the value of the expression that was validated, it becomes
possible to rewrite the following code:

this.foo = foo;
this.var = bar;

into the following form

this.foo = precond.checkIsArray(foo);
this.bar = precond.checkIsFunction(bar);

which makes the precondion checks clearer and a lot more compact.

The updated methods are:
- checkIsDef
- checkIsDefAndNotNull
- checkIsArray
- checkIsNumber
- checkIsBoolean
- checkIsFunction
- checkIsObject