RE: constant `e'

[Drew Adams]

> I could consider renaming it (later) if someone suggests a good new
> name.


`numeric-constant-e', and adopt that (or similar) as the convention for
numeric constants: `numeric-constant-pi', etc.

However, I still prefer, in descending order or preference:

1. (exp 1) ; that is, drop the variable altogether
2. (e); that is, make it a function that does (exp 1) or picks up the
memoized value after doing (exp 1 once).