Change custom section names to string slice

We will never need to create these names dynamically.
e.g. "reloc.CODE", "reloc.DATA", and "linking" will all
come from specific places in our backend code.
It's a different story for function names etc.

compiler/gen_wasm/src/module_builder.rs

@@ -185,7 +185,7 @@ impl Serialize for RelocationEntry {
 }
 
 pub struct RelocationSection<'a> {
-    pub name: String,
+    pub name: &'a str,
     pub entries: &'a Vec<'a, RelocationEntry>,
 }