Introduce FnSuccess/FnSuccessF shortcut functions

This allows to clean up a lot of buffer allocation in
individual functions. As a separate benefit, FnSuccessF
lifts length restriction which was imposed on scalar
sizes returned from functions.
Add shortcut function FnFailure
Introduce shortcut functions FnSuccess{Bool,True,False}

Those shortcuts eliminate common and noisy construct
if (something)
{
    return FnSuccess("any");
}
else
{
    return FnSuccess("!any");
}
Rename FnSuccess -> FnReturn, FnReturnBool -> FnReturnContext
Remove FnReturnContext{True,False}, they are not shorter than bodies

FnSuccess{False,True} was shorter than FnSuccessBool({false,true}),
but after renaming it is no longer the case.
Merge pull request #1207 from dottedmag/refactoring/simplify-fncallresult

Refactoring/simplify FnCallResult