Restores ability to define host functions w/o context via reflection (#832)

This restores the ability to leave out the initial context parameter
when defining functions with reflection. This is important because some
projects are porting from a different library to wazero, and all the
alternatives are not contextualized.

For example, this project is porting envoy host functions, and the
original definitions (in mosn) don't have a context parameter. By being
lenient, they can migrate easier.

See 6b813482b6/pkg/proxywasm/wazero/imports_v1.go

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/wasm/function_definition_test.go

@@ -1,7 +1,6 @@
 package wasm
 
 import (
-	"context"
 	"testing"
 
 	"github.com/tetratelabs/wazero/api"
@@ -10,7 +9,7 @@ import (
 
 func TestModule_BuildFunctionDefinitions(t *testing.T) {
 	nopCode := &Code{Body: []byte{OpcodeEnd}}
-	fn := func(context.Context) {}
+	fn := func() {}
 	tests := []struct {
 		name            string
 		m               *Module