Rename TegVar?

[dromaiidae]

This could be slightly confusing to a new user

TegVars currently are for variables that are integrated in a Teg expression at some point; Vars are for all other variables
But Teg is also the name of this language, so a TegVar and a Var in Teg sound like the same thing

This seems fairly bug-prone for beginners; rename?

[martinjm97]

Remove TegVar and implement the appropriate static analysis to accommodate for the change