Add parser/IR support for soft-maintain

[dcerys]

A slight variation of maintain

soft-maintain [Not yet supported] [Not yet supported by parser and IR]

• Description: Execute the statement fn one time. While doing so, the condition cond-expr can be violated. However, at the completion of executing the statement fn, the condition cond-expr must be true.
• Note the distinction of the duration and frequency in which the condition is checked among the unless, when, whenever, and maintain statements.
• Syntax: '(' 'soft-maintain' cond-expr opt-bounds? fn ')'
• Example:

(soft-maintain (= door "open") :bounds [2 30]
     (do-lots-of-stuff-possibly-involving-the-door))