Lambda fields should not be hardcoded in Emit.hs

[scolsen]

as mentioned in #1391 the Lambda type having a field called delete is problematic for C++ interop. Since the struct is defined in core.h, a user using a c++ compiler could in theory rename the field and solve the problem, unfortunately it won't because, the field is hardcoded in Emit.hs.

Instead, we should register the lambda type from core and use its binding information to emit C in the emitter (vs. hardcoding).

[eriksvedang]

Yes, let's try that.