compiler: include type information about the runtime in the compiler

These types are often known to the compiler already. Moving them into
the compiler is an important step for two related reasons:

  * It makes the compiler better testable. Together with
    https://github.com/tinygo-org/tinygo/pull/1008 it will make it
    possible to test the compiler without having to load the runtime
    package.
  * It makes it easier to compile packages independently as the type
    information of the runtime package doesn't need to be present.

I had to use hack to get this to work well: internal/task.Task is now an
opaque struct. This is necessary because there is a dependency from
*runtime.channel -> *runtime.channelBlockedList -> *internal/task.Task.
I don't want to include the definition of the internal/task.Task struct
in the compiler directly as that would make changing the internal/task
package a lot harder and the compiler doesn't need to know the layout of
that struct anyway.

compiler/calls.go

@@ -4,7 +4,6 @@ import (
 	"go/types"
 	"strconv"
 
-	"golang.org/x/tools/go/ssa"
 	"tinygo.org/x/go-llvm"
 )
 
@@ -35,11 +34,9 @@ const (
 
 // createCall creates a new call to runtime.<fnName> with the given arguments.
 func (b *builder) createRuntimeCall(fnName string, args []llvm.Value, name string) llvm.Value {
-	fn := b.program.ImportedPackage("runtime").Members[fnName].(*ssa.Function)
-	llvmFn := b.getFunction(fn)
-	if llvmFn.IsNil() {
-		panic("trying to call non-existent function: " + fn.RelString(nil))
-	}
+	llvmFn := b.getFunctionRaw(b.getRuntimeFuncType(fnName), functionInfo{
+		linkName: "runtime." + fnName,
+	})
 	args = append(args, llvm.Undef(b.i8ptrType))            // unused context parameter
 	args = append(args, llvm.ConstPointerNull(b.i8ptrType)) // coroutine handle
 	return b.createCall(llvmFn, args, name)