Function: sort_text

--Usage--
> sort_text(order: order specification, some_text)
> sort_rule(order: order specification)(some_text)

Sort a string or filter it by keeping specific characters.

When the @order@ parameter is not given, the input is sorted in alphabetical order.

Otherwise the @order@ parameter determines what characters to show in the output.
The @order@ is a string consisting of multiple ''parts'', each part selects a specific thing from the input.
! Part		<tt>sort("zzyyxx")</tt>	Description
| @"x"@		@"xx"@			A normal character, selects all matching characters from the input
| @"<wxy>"@	@"xy"@			Selects the characters from the input, but only once.
		 			@"<xy>"@ is the same as @"<x><y>"@.
| @"[wxy]"@	@"yyxx"@		Selects the given characters, but keep them in the same order as in the input.
| @"(xwz)"@	@"zzxx"@		Selects the given characters, and put them in the order that minimizes the distance.
		 			around a 'circle'. Imagine w x and y in a circle, then "zx" is shorter then "x.z" because the latter passes the "w" space.
		 			This sorting option is used to sort colored mana symbols in Magic.

This function is available in [[script:rule form]].

--Parameters--
! Parameter	Type				Description
| @input@	[[type:string]]			Text to sort
| @order@	[[type:string]] (optional)	Order specification

--Examples--
> sort_text("banana")                 ==  "aaabnn"
> sort_text(order: "na",   "banana")  ==  "nnaaa"
> sort_text(order: "<na>", "banana")  ==  "na"
> sort_text(order: "[na]", "banana")  ==  "anana"
> sort_text(order: "[1234567890](wubrg)", "21wg")  ==  "21gw"
>
> f := sort_rule(order: "[1234567890](wubrg)")
> f("21wg")  ==  "21gw"