Allow declaring constants

[skx]

Related to #19 we could move PI, & E, into the monkey-based init-file(s) if we had the ability to define constants, which couldn't be modified.

Something like:

const PI=3.1.4..

Instead of:

let PI=3.14..

This should be a simple matter of updating our environment.

[skx]

At this point we have a const expression which works - but scoping is subtly broken. Consider this code:

 let a = 333;
 function foo() {
     const a = 33;
     puts("A:", a, "\n");
 }
 foo()
 puts( "A:" , a, "\n");

The code outputs 333, 3333. This is because of the way we lookup the values of constants and variables separately. Instead the should do it once, following scoped-rules.