Rewrite linear memory handling in terms of simple mmap/VirtualAlloc.

The memmap crate doesn't make it straightforward to have part of the
region be writeable and part readonly. Since this is a fairly boutique
use case, and we don't need all that much code, just use the low-level
APIs directly.

Also, introduce a concept of "tunables" for adjusting the parameters of
the runtime.

lib/environ/src/lib.rs

@@ -42,11 +42,19 @@ extern crate alloc;
 mod compilation;
 mod environ;
 mod module;
+mod tunables;
 mod vmcontext;
 
 pub use compilation::{compile_module, Compilation, Relocation, RelocationTarget, Relocations};
 pub use environ::{ModuleEnvironment, ModuleTranslation};
-pub use module::{DataInitializer, Export, Module, TableElements};
+pub use module::{DataInitializer, Export, MemoryPlan, MemoryStyle, Module, TableElements};
+pub use tunables::Tunables;
+
+/// WebAssembly page sizes are defined to be 64KiB.
+pub const WASM_PAGE_SIZE: u32 = 0x10000;
+
+/// The number of pages we can have before we run out of byte index space.
+pub const WASM_MAX_PAGES: u32 = 0x10000;
 
 #[cfg(not(feature = "std"))]
 mod std {