Implement support for constants

[bobbimanners]

Would be nice to be able to define constants and use them in constant expressions.

For example

const size=10
word A[size*size] = {}
word i=0
for i=1:size
 A[i]=1
endfor

However we are super short of memory so we may need to implement this in a devious way.

[bobbimanners]

Maybe this is a 'cheaper' way to implement this feature:

word _size=10

Currently variable names must begin with alpha character. We could use a leading underscore to mark a constant.

[bobbimanners]

Maybe with trailing _ too?

word _size_=10; ' Looks better?

[bobbimanners]

Implemented in v0.66 (with nicer syntax - see docs!)