Find better new symbol for `;` (relational composition)

[jamesmckinna]

Responding to my query on #2301 , @Taneb wrote:

`;` is (according to agda-mode's "Information about the character at point" tool) a Greek question mark, which is written with `\;`, but there's a dozen characters with that and it's the 11th in the list.

I don't think we should be using that character, it's identical to the semicolon graphically (it's even normalized to the semicolon by Unicode) and bound to cause confusion. But I don't have a suggestion for what to use instead. EDIT: `⨾` (first in the list of `\;` but also \z;`) is "Z NOTATION RELATION COMPOSITION" per the Unicode standard. So it's at least semantically appropriate

Originally posted by @Taneb in #2301 (comment)

I too had queried that info, finding that the symbol is written with \;, but not as far as finding it's the 11th in the list.

I would prefer to use the symbol which exhibits REL as the prototypical bicategory, namely $\otimes$, but I have had push back for suggesting that usage in other settings in the past... but here I think it does make sense, and ushers in the use of the left- and right- linear lollipops for the corresponding adjoints...