Demand: "Better" syntax for demand signatures

@simonpj said that he doesn't quite like the current syntax (since #18903 (closed), see also the updated entry for -fstrictness in the user's guide on master) of demand signatures, in particular the abbreviations of Cards and the fact that we now prefix product demands with P (e.g. SP(SU,A)).

Abbreviation for Cards

The current abbreviations for Cards are

interval	set of denoted cardinalities	ppr syntax	explanation tying syntax to semantics
[1,0]	{}	B	bottom
[0,0]	{0}	A	absent
[0,1]	{0,1}	1	used at most once
[0,n]	{0,1,n}	U	top, no information, used at least 0 and at most n times
[1,1]	{1}	S	strict, used exactly once
[1,n]	{1,n}	M	"multi-strict", used at least once, but possibly many times

Simon argued that

• he always mistakes 1 to mean "exactly once" rather than "at most once" (The only reason I chose 1 here in the first place was that usage demands prior to #18903 (closed) used it that way)
• then thinks that S is a better fit for M
• U should really be Lazy. (similarly, I chose U because it was the top element in the usage lattice, no particular reason.)

His suggestions leave open how to name "at most once" then. He proposed F for "few", but I'm not satisfied with that.

So, in summary, we have

interval	set of denoted cardinalities	ppr syntax	explanation tying syntax to semantics
[1,0]	{}	B	bottom
[0,0]	{0}	A	absent
[0,1]	{0,1}	M	used at most once, "Maybe"
[0,n]	{0,1,n}	L	top, no information, used at least 0 ("lazy") and at most n times
[1,1]	{1}	1	used exactly once
[1,n]	{1,n}	S	used at least once, but possibly many times

Product sub-demands

Simon proposed to omit the P in a demand like SP(SU,A) --> S(SU,A). But if we omit the P in call sub-demands, in which product sub-demands occur without the leading evaluation cardinality of the surrounding demand, look a bit weird because of the doubly nested parentheses: C1(P(SU,A)) --> C1((SU,A)). We can't omit them, otherwise C1(SU) is ambiguous with C1(P(SU)).

Maybe the doubly nested parentheses are OK.

Summary

We need to settle on a notation for cardinalities, where the "used at most once" slot is contested the most, and also decide whether we want to drop the P in product sub-demands, which means making peace with doubly nested parentheses C1((SU,A)).

The discussion ultimately settled for M ("Maybe") and not omitting the P.