Reimplement the C# API.

This commit reimplements the C# API in terms of a Wasmtime linker.

It removes the custom binding implementation that was based on reflection in
favor of the linker's implementation.

This should make the C# API a little closer to the Rust API.

The `Engine` and `Store` types have been hidden behind the `Host` type which is
responsible for hosting WebAssembly module instances.

Documentation and tests have been updated.

crates/misc/dotnet/tests/Modules/FunctionThunking.wat

@@ -1,20 +1,24 @@
 (module
- (type $FUNCSIG$iii (func (param i32 i32) (result i32)))
- (type $FUNCSIG$v (func))
- (import "env" "add" (func $add (param i32 i32) (result i32)))
- (import "env" "do_throw" (func $do_throw))
- (table 0 anyfunc)
- (memory $0 1)
- (export "memory" (memory $0))
- (export "add_wrapper" (func $add_wrapper))
- (export "do_throw_wrapper" (func $do_throw_wrapper))
- (func $add_wrapper (; 2 ;) (param $0 i32) (param $1 i32) (result i32)
-  (call $add
-   (get_local $0)
-   (get_local $1)
-  )
+ (import "env" "add" (func $env.add (param i32 i32) (result i32)))
+ (import "env" "swap" (func $env.swap (param i32 i32) (result i32 i32)))
+ (import "env" "do_throw" (func $env.do_throw))
+ (import "env" "check_string" (func $env.check_string (param i32 i32)))
+ (memory (export "mem") 1)
+ (export "add" (func $add))
+ (export "swap" (func $swap))
+ (export "do_throw" (func $do_throw))
+ (export "check_string" (func $check_string))
+ (func $add (param i32 i32) (result i32)
+  (call $env.add (local.get 0) (local.get 1))
  )
- (func $do_throw_wrapper (; 3 ;)
-  (call $do_throw)
+ (func $swap (param i32 i32) (result i32 i32)
+  (call $env.swap (local.get 0) (local.get 1))
  )
+ (func $do_throw
+  (call $env.do_throw)
+ )
+ (func $check_string
+  (call $env.check_string (i32.const 0) (i32.const 11))
+ )
+ (data (i32.const 0) "Hello World")
 )