[Nickle]Const in nickle

[Keith Packard]

I've ripped out the strange const stuff I did yesterday and implemented 
something simple -- const is a new storage class equivalent in lifetime to 
global but which is not addressable.  This means the value cannot be 
changed nor can a reference be generated to it.

	> const pi = 3.141592653589793
	3.141592653589793
	> pi = 7
	->     pi = 7
	Invalid use of const "pi"
	> &pi
	->     &pi
	Invalid use of const "pi"
	> 

I think this covers essentially all "reasonable" uses of 'const' while 
keeping things as simple as possible.

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