Further compress the in-memory representation of address maps (#2324)

This commit reduces the size of `InstructionAddressMap` from 24 bytes to
8 bytes by dropping the `code_len` field and reducing `code_offset` to
`u32` instead of `usize`. The intention is to primarily make the
in-memory version take up less space, and the hunch is that the
`code_len` is largely not necessary since most entries in this map are
always adjacent to one another. The `code_len` field is now implied by
the `code_offset` field of the next entry in the map.

This isn't as big of an improvement to serialized module size as #2321
or #2322, primarily because of the switch to variable-length encoding.
Despite this though it shaves about 10MB off the encoded size of the
module from #2318

crates/debug/src/transform/expression.rs

@@ -1145,14 +1145,21 @@ mod tests {
                     InstructionAddressMap {
                         srcloc: SourceLoc::new(code_section_offset + 12),
                         code_offset: 5,
-                        code_len: 3,
+                    },
+                    InstructionAddressMap {
+                        srcloc: SourceLoc::default(),
+                        code_offset: 8,
                     },
                     InstructionAddressMap {
                         srcloc: SourceLoc::new(code_section_offset + 17),
                         code_offset: 15,
-                        code_len: 8,
                     },
-                ],
+                    InstructionAddressMap {
+                        srcloc: SourceLoc::default(),
+                        code_offset: 23,
+                    },
+                ]
+                .into(),
                 start_srcloc: SourceLoc::new(code_section_offset + 10),
                 end_srcloc: SourceLoc::new(code_section_offset + 20),
                 body_offset: 0,