all: refactor compile options

Move most of the logic of determining which compiler configuration to
use (such as GOOS/GOARCH, build tags, whether to include debug symbols,
panic strategy, etc.) into the compileopts package. This makes it a
single source of truth for anything related to compiler configuration.

It has a few advantages:

  * The compile configuration is independent of the compiler package.
    This makes it possible to move optimization passes out of the
    compiler, as they don't rely on compiler.Config anymore.
  * There is only one place to look if an incorrect compile option is
    used.
  * The compileopts provides some resistance against unintentionally
    picking the wrong option, such as with c.selectGC() vs c.GC() in the
    compiler.
  * It is now a lot easier to change compile options, as most options
    are getters now.

compiler/func.go

@@ -34,7 +34,7 @@ const (
 func (c *Compiler) funcImplementation() funcValueImplementation {
 	// Always pick the switch implementation, as it allows the use of blocking
 	// inside a function that is used as a func value.
-	switch c.selectScheduler() {
+	switch c.Scheduler() {
 	case "coroutines":
 		return funcValueSwitch
 	case "tasks":