all: remove golang.org/x/sys/windows usages (or give up Go 1.16)

[hajimehoshi]

What feature would you like to be added?

Now Ebitengine uses golang.org/x/sys@v0.0.0-20220818161305-2296e01440c6. I tried to update this to v0.1.0 but failed due to this error (b752a6c):

Error: ../../../go/pkg/mod/golang.org/x/sys@v0.1.0/windows/syscall.go:84:16: undefined: unsafe.Slice
Error: ../../../go/pkg/mod/golang.org/x/sys@v0.1.0/windows/syscall_windows.go:141:29: undefined: unsafe.Slice

unsafe.Slice was introduced at Go 1.17. However, I want to keep supporting Go 1.16 as gccgo still uses Go 1.16.

Then, I'm now considering removing golang.org/x/sys/windows usages.

Why is this needed?

We will be able to update golang.org/x/sys without breaking compatibility with Go 1.16.

[hajimehoshi]

https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libgo/VERSION;h=39560f0fcdd63e4bb8eead6a3f849e91636d31eb;hb=2ee5e4300186a92ad73f1a1a64cb918dc76c8d67

gccgo12 supports Go 1.18!

[hajimehoshi]

We have decided to drop Go 1.16.