all: refactor reflect package

This is a big commit that changes the way runtime type information is stored in
the binary. Instead of compressing it and storing it in a number of sidetables,
it is stored similar to how the Go compiler toolchain stores it (but still more
compactly).

This has a number of advantages:

  * It is much easier to add new features to reflect support. They can simply
    be added to these structs without requiring massive changes (especially in
    the reflect lowering pass).
  * It removes the reflect lowering pass, which was a large amount of hard to
    understand and debug code.
  * The reflect lowering pass also required merging all LLVM IR into one
    module, which is terrible for performance especially when compiling large
    amounts of code. See issue 2870 for details.
  * It is (probably!) easier to reason about for the compiler.

The downside is that it increases code size a bit, especially when reflect is
involved. I hope to fix some of that in later patches.

compiler/testdata/defer-cortex-m-qemu.ll

@@ -4,7 +4,7 @@ target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
 target triple = "thumbv7m-unknown-unknown-eabi"
 
 %runtime.deferFrame = type { ptr, ptr, [0 x ptr], ptr, i1, %runtime._interface }
-%runtime._interface = type { i32, ptr }
+%runtime._interface = type { ptr, ptr }
 %runtime._defer = type { i32, ptr }
 
 declare noalias nonnull ptr @runtime.alloc(i32, ptr, ptr) #0