Renamed "always :{...}:" to "always ~{...}~".

Such syntactic change anticipates our future introduction of split aliases
(i.e., alias-like runnables split into low-level aliases leveraging ":{" and
"}:" delimiters).
Drafted split alias implementation.

:define_split_alias() now defines a so-called split alias (e.g., alias
accepting an arbitrary number of arguments). While obscenely interesting, such
definitions are probably bathetically fragile. (We shall hammer this thereupon!)