While adding some code to clear the Next field when popping from a task stack for safety reasons, the clear was placed outside of a nil pointer check.
As a result, (*internal/task.Stack).Pop panicked when the Stack is empty.
This is the kind that is used in Go (actually CGo) for exporting
functions. I think it's best to use //export instead of our custom
//go:export pragma, for consistency (they are equivalent in TinyGo).
Therefore I've updated all instances to the standard format (except for
two that are updated in https://github.com/tinygo-org/tinygo/pull/1024).
No smoke tests changed (when comparing the output hash), except for some
wasm tests that include DWARF debug info and tend to be flaky anyway.
This adds support for the `-scheduler=tasks` flag for AVR. On most AVR
chips you wouldn't want to run a real scheduler but it may be useful in
some cases, especially on devices with more RAM. It is disabled by
default.
