Use period as prefix for invisible operators

[runarberg]

BREAKING CHANGE

The following operators have been changed from old ⇒ new

• INVISIBLE PLUS: &+ ⇒ .+
• INVISIBLE TIMES: &* ⇒ .*
• FUNCTION APPLICATION: &$ ⇒ .$
• INVISIBLE SEPARATOR: &, ⇒ .,

Resolves: #75