wasmtime: Implement table.get and table.set

These instructions have fast, inline JIT paths for the common cases, and only
call out to host VM functions for the slow paths. This required some changes to
`cranelift-wasm`'s `FuncEnvironment`: instead of taking a `FuncCursor` to insert
an instruction sequence within the current basic block,
`FuncEnvironment::translate_table_{get,set}` now take a `&mut FunctionBuilder`
so that they can create whole new basic blocks. This is necessary for
implementing GC read/write barriers that involve branching (e.g. checking for
null, or whether a store buffer is at capacity).

Furthermore, it required that the `load`, `load_complex`, and `store`
instructions handle loading and storing through an `r{32,64}` rather than just
`i{32,64}` addresses. This involved making `r{32,64}` types acceptable
instantiations of the `iAddr` type variable, plus a few new instruction
encodings.

Part of #929

cranelift/wasm/src/environ/dummy.rs

@@ -22,6 +22,7 @@ use cranelift_codegen::ir::types::*;
 use cranelift_codegen::ir::{self, InstBuilder};
 use cranelift_codegen::isa::TargetFrontendConfig;
 use cranelift_entity::{EntityRef, PrimaryMap, SecondaryMap};
+use cranelift_frontend::FunctionBuilder;
 use std::boxed::Box;
 use std::string::String;
 use std::vec::Vec;
@@ -452,17 +453,17 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
 
     fn translate_table_get(
         &mut self,
-        mut pos: FuncCursor,
+        builder: &mut FunctionBuilder,
         _table_index: TableIndex,
         _table: ir::Table,
         _index: ir::Value,
     ) -> WasmResult<ir::Value> {
-        Ok(pos.ins().null(self.reference_type()))
+        Ok(builder.ins().null(self.reference_type()))
     }
 
     fn translate_table_set(
         &mut self,
-        _pos: FuncCursor,
+        _builder: &mut FunctionBuilder,
         _table_index: TableIndex,
         _table: ir::Table,
         _value: ir::Value,