Support "casting" the return value of custom functions into constants.

Support "casting" the return value of custom functions into constants.

A custom function is one that is defined by AddFunction.  If the function
is defined as:

    AddFunction FuncName asValue=1 { ... }

then the function is evaluated normally, then typecast to a constant as
follows:

(1) If the return value is a time span (a "boolean" value), then if the
    current time of the simulation is within the time span, then return 1,
    or 0 otherwise.

(2) If the return value is a linear function, then if the current time of
    the simulation is within the function's domain, then the function is
    simply evaluated at the current time, or 0 otherwise.

(3) If the return value is an action, then return 1 if the action is off
    of cooldown, or 0 if it is on cooldown.

git-svn-id: svn://svn.curseforge.net/wow/ovale/mainline/trunk@1080 d5049fe3-3747-40f7-a4b5-f36d6801af5f