Use function pointers instead of macro-unrolled loops in rustc_query_impl

By making these standalone functions, we
a) allow making them extensible in the future with a new `QueryStruct`
b) greatly decrease the amount of code in each individual function, avoiding exponential blowup in llvm

compiler/rustc_query_impl/src/lib.rs

@@ -1,6 +1,8 @@
 //! Support for serializing the dep-graph and reloading it.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+// this shouldn't be necessary, but the check for `&mut _` is too naive and denies returning a function pointer that takes a mut ref
+#![feature(const_mut_refs)]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(once_cell)]