Add waitForAnyKey method

[richardpawson]

Press any key to continue... is a common requirement in early programming. Currently you need to write e.g.:

let k be inputString("Press Enter to continue")

which is awkward because:

• The identifier k is never used
• It has to be Enterbecause it is an input statement - any other key would need to be followed by Enter

So with this method you just write e.g.

print "Press any key to continue...`
call waitForAnyKey()

[richardpawson]

Implement and working (as part, originally, of #849) - but needs tests and documentation still.