Removes duplicate code in src/obj.rs, crates/obj and crates/jit/object.rs (#1993)

Changes:

 -  Moves object creation code from crates/jit/object.rs to the creates/obj (as ObjectBuilder)
 -   Removes legacy crates/obj/function.rs
 -  Removes write_debugsections

crates/environ/src/data_structures.rs

@@ -1,10 +1,10 @@
 #![doc(hidden)]
 
 pub mod ir {
-    pub use cranelift_codegen::binemit::Stackmap;
+    pub use cranelift_codegen::binemit::{Reloc, Stackmap};
     pub use cranelift_codegen::ir::{
-        types, AbiParam, ArgumentPurpose, Signature, SourceLoc, StackSlots, TrapCode, Type,
-        ValueLabel, ValueLoc,
+        types, AbiParam, ArgumentPurpose, JumpTableOffsets, LibCall, Signature, SourceLoc,
+        StackSlots, TrapCode, Type, ValueLabel, ValueLoc,
     };
     pub use cranelift_codegen::{ValueLabelsRanges, ValueLocRange};
 }