Add runtime error when attempting to unpack a non unpackable Ast node

Ex.:

```
Error unpacking a non unpackable Ast node on `$(binary_xor ...? {` at line 29 with context: [
    "^"
]

Hint: use a non ellipsis expansion as in `$(binary_xor ? {`
```

cc ircmaxell/php-compiler#29

src/Expansion.php

@@ -230,6 +230,16 @@ private function mutate(TokenStream $ts, Ast $context, Engine $engine) : TokenSt
 
                     $context = $context->unwrap();
 
+                    if (! is_array($context))
+                        $this->fail(
+                            "Error unpacking a non unpackable Ast node on `$(%s ...%s {` at line %d with context: %s\n\n%s",
+                            $result->{'* label name'}->token(),
+                            $result->optional,
+                            $result->{'* label name'}->token()->line(),
+                            json_encode([$context], self::PRETTY_PRINT),
+                            sprintf("Hint: use a non ellipsis expansion as in `$(%s %s {`", $result->{'* label name'}->token(), $result->optional)
+                        );
+
                     $delimiters = $result->{'delimiters'};
 
                     // normalize associative arrays

tests/phpt/issues/ircmaxell-php-compiler#29.phpt

@@ -0,0 +1,51 @@
+--TEST--
+Test for bug found at https://github.com/ircmaxell/php-compiler/pull/29 --pretty-print
+--FILE--
+<?php
+
+$(macro :unsafe :recursive) {
+    $(optional(buffer('unsigned')) as unsigned) compile {
+        $(
+            repeat(
+                either(
+                    chain(
+                        T_VARIABLE as result,
+                        token('='),
+                        either(
+                            chain(
+                                T_VARIABLE as binary_left,
+                                either(token('^') as binary_xor) as binary_op,
+                                either(T_VARIABLE as binary_variable, T_LNUMBER as binary_number) as binary_right
+                            ) as binary
+                        ),
+                        token(';')
+                    ) as assignop
+                )
+            ) as stmts
+        )
+    }
+} >> {
+    $(stmts ... {
+        $(assignop ? ... {
+            $(binary ? ... {
+                $(binary_op ... {
+                    $(binary_xor ? ... {
+                        $(result) = $this->context->builder->bitwiseXor($(binary_left), $__right);
+                    })
+                })
+            })
+        })
+    })
+}
+
+compile {
+    $result = $value ^ 1;
+}
+
+?>
+--EXPECTF--
+Error unpacking a non unpackable Ast node on `$(binary_xor ...? {` at line 29 with context: [
+    "^"
+]
+
+Hint: use a non ellipsis expansion as in `$(binary_xor ? {`