Compiler abort on unit as map key

[simeonmiteff]

The following program:

module test;

type Key = unit {a: uint8;};

global m: map<Key, uint8>;

m[[$a=1]] = 1;

print m;

...causes spicy to abort with a failed assertion:

$ spicyc -j assert_fails_map_key_types.spicy 
spicyc: /home/simeon/tmp/spicy/hilti/toolchain/src/compiler/codegen/types.cc:673: 
hilti::detail::visitor::Visitor<hilti::detail::codegen::CxxTypes, {anonymous}::VisitorStorage, hilti::Node, 
hilti::detail::visitor::Order::Pre>::result_t {anonymous}::VisitorStorage::operator()(const hilti::type::Struct&, 
hilti::detail::visitor::Visitor<hilti::detail::codegen::CxxTypes, {anonymous}::VisitorStorage, hilti::Node, 
hilti::detail::visitor::Order::Pre>::position_t): Assertion `typeID(p.node)' failed.
Aborted (core dumped)

This may be a duplicate of #1233 (i.e., perhaps this is expected but not checked for by the parser and documented as unsupported).

[rsmmr]

Like #1238, will need some digging what's going on.