Lift requirement that bytes forwarded from filter be mutable.

Since we did not declare the `bytes` argument to `unit::forward`
constant it was implicitly mutable so that it was impossible to e.g.,
pass literal bytes. This patch uses the originally intended type.

Closes #1659.

doc/autogen/types/unit.rst

@@ -33,7 +33,7 @@
     starting position can also be explicitly given, but must lie inside
     the same region.
 
-.. spicy:method:: unit::forward unit forward False void (inout data: bytes)
+.. spicy:method:: unit::forward unit forward False void (data: bytes)
 
     If the unit is connected as a filter to another one, this method
     forwards transformed input over to that other one to parse. If the

spicy/toolchain/include/ast/operators/unit.h

@@ -364,7 +364,7 @@ BEGIN_METHOD(unit, Forward)
             hilti::operator_::Signature{.self = hilti::type::constant(spicy::type::Unit(type::Wildcard())),
                                         .result = hilti::type::void_,
                                         .id = "forward",
-                                        .args = {{"data", hilti::type::Bytes()}},
+                                        .args = {{"data", hilti::type::constant(hilti::type::Bytes())}},
                                         .doc = R"(
 If the unit is connected as a filter to another one, this method forwards
 transformed input over to that other one to parse. If the unit is not connected,