syntax-change: remove `return`

[nikodemus]

Replace return x with x! -- consistent with using ! to terminate definitions.

Implementation plan: set precedence so that ! is consumed by sequence parsing, and wraps the prefix in a return. Allow returning from define, allowing guard clauses in defines.

define MyDef
   OtherDef > MagicValue
      ifTrue: { 0! }
   ...
   computedMagicValue!

Especially useful once main module is able to pass static assets to imported modules.