modernize syntax

[karenetheridge]

• $foo equ 'bar' instead of ($foo//'') eq 'bar' (Syntax::Operator::Equ)
• postfix deref (requires 5.20)
• hash-context hash slices and array slices (requires 5.20)
• sub signatures (requires 5.20)

[karenetheridge]

This is done in v0.525, except equ which is not possible until pluggable infix operators are added to core.