Implement the pooling instance allocator.

This commit implements the pooling instance allocator.

The allocation strategy can be set with `Config::with_allocation_strategy`.

The pooling strategy uses the pooling instance allocator to preallocate a
contiguous region of memory for instantiating modules that adhere to various
limits.

The intention of the pooling instance allocator is to reserve as much of the
host address space needed for instantiating modules ahead of time and to reuse
committed memory pages wherever possible.

crates/environ/src/vmoffsets.rs

@@ -50,6 +50,7 @@ fn align(offset: u32, width: u32) -> u32 {
 
 /// This class computes offsets to fields within `VMContext` and other
 /// related structs that JIT code accesses directly.
+#[derive(Debug, Clone, Copy)]
 pub struct VMOffsets {
     /// The size in bytes of a pointer on the target.
     pub pointer_size: u8,