Syntax updates and snippet additions

• Remove semicolon from the fn snippet
• add a snippet to create a tests module
• add opt-out trait impl syntax
• Support next/prev with multiple views
• Hide, don't delete messages when dismissed.
• Syntax: More stringent escape handling.
• • Allow _ in unicode escapes.
• • Reject byte/char literals with too many chars.
• • Reject non-ascii in byte literals.
• • Reject ascii escape with >0x7f.
• • Reject bad escape sequences.