return in contracts have undesirable effects, consider a change

[vssukharev]

Let we have such code:

import std::collections::list;
def IntList = List(<int>);
<*
 @require {| IntList* l = (IntList*) data; return l.allocator && l.capacity; |}
*>
fn void f(void* data)
{
  // ...
}

It fails compilation with the following error:

 5: def IntList = List(<int>);
 6: 
 7: <*
 8:  @require {| IntList* l = (IntList*) data; return l.allocator && l.capacity; |}
                                                     ^
(/home/starleks/coding/probe/c3-sandbox/main.c3:8:50) Error: Expected ';'

[lerno]

This is due to return being handled as a token inside of contracts.

We could possibly replace this with something else, such as:

$return
$return()
$$return
$$RETURN
$RETURN
@return
$ret
$ret()
$$RET
$RET
@ret
$_
$$_

[lerno]

<*
 @ensure $return() > 0
*>
fn int foo() { ... }

<*
 @ensure $ret() > 0
*>
fn int foo() { ... }

<*
 @ensure $_ > 0
*>
fn int foo() { ... }

[lerno]

An advantage would be that the lexer would no longer need to special case lexing for return inside of the <* *>

[vssukharev]

We have already $vaarg which is context-dependent compile time variable. Technically, we could explain the reason for $return or $ret the same.