Update README.md instructions for clarity

... also plaintext readability.
cexp: add int64 to value set, intro. cexp data type

Since the language now has non-policy values, there are opportunities
for type errors. In exchange the language now supports named constants,
though as of this commit, programs cannot use them effectively.
(Currently using an identifier to which a program has assigned a
constant will ensure that program produces a type error.)

The cexp data type is included to support additional expressions in the
future.
cexp: perform substitution in actions

Perform substition of constants in actions during compilation. The
parser stores the cexp expressions with a function that will produce an
action given integer constants. Since cexp may be an identifer, this
application has to wait until compile time.

Here's an example program (modified from examples/guide/tut1.nc) that
works:

    let repeater =
      if inPort = 1 then (let compaq = 2 in fwd(compaq))
      else fwd(1) in
    monitorTable(1, repeater)

Warnings about type errors from the previous commit still apply. They're
multiplying.
Merge pull request #109 from seliopou/cexp

Some hacks by Spiros