Don't copy executable code into a CodeMemory (#3265)

* Don't copy executable code into a `CodeMemory`

This commit moves a copy from compiled artifacts into a `CodeMemory`. In
general this commit drastically changes the meaning of a `CodeMemory`.
Previously it was an iteratively-pushed-on structure that would
accumulate executable code over time. Afterwards, however, it's a
manager for an `MmapVec` which updates the permissions on text section
to ensure that the pages are executable.

By taking ownership of an `MmapVec` within a `CodeMemory` there's no
need to copy any data around, which means that the `.text` section in
the ELF image produced by Wasmtime is usable as-is after placement in
memory and relocations have been resolved. This moves Wasmtime one step
closer to being able to directly use a module after it's `mmap`'d into
memory, optimizing when a module is loaded.

* Fix windows section alignment

* Review comments

crates/jit/src/lib.rs

@@ -30,10 +30,9 @@ mod unwind;
 
 pub use crate::code_memory::CodeMemory;
 pub use crate::instantiate::{
-    finish_compile, subslice_range, CompiledModule, CompiledModuleInfo, ModuleCode, SetupError,
+    finish_compile, subslice_range, CompiledModule, CompiledModuleInfo, SetupError,
     SymbolizeContext, TypeTables,
 };
-pub use crate::link::link_module;
 pub use crate::mmap_vec::MmapVec;
 pub use profiling::*;