Deduplicate SoC/arch-specific task switching related code

[bugadani]

Github handles moved files a bit funnily but I can't do much about that. This PR extract the architecture-common parts out of the old timer_*.rs files, moves those timer files into a folder. These files still do more than just timer-related setup and interrupt handling, but I didn't actually want to change how the big picture looks.

I un-public-ed some modules that were hidden. I'm not sure if some dependency relied on these modules being public, but hidden docs usually indicate that is not an intended use.

[bjoernQ]

Looks good! I also meant to do some cleanup there but never got to it

There are however some unused imports - e.g.

warning: unused imports: `Alarm`, `Periodic`, `SystemTimer`, `Target`, `riscv`, `self`
 --> esp-wifi\src\timer\timer_esp32c3.rs:5:23
  |
5 |         peripherals::{self, Interrupt},
  |                       ^^^^
6 |         prelude::*,
7 |         riscv,
  |         ^^^^^
8 |         systimer::{Alarm, Periodic, SystemTimer, Target},
  |                    ^^^^^  ^^^^^^^^  ^^^^^^^^^^^  ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused import: `binary`
 --> esp-wifi\src\timer\riscv.rs:6:5
  |
6 |     binary,
  |     ^^^^^^


warning: unused imports: `TIMG1`, `Timer0`, `Timer`
 --> esp-wifi\src\timer\timer_esp32.rs:4:25
  |
4 |     peripherals::{self, TIMG1},
  |                         ^^^^^
5 |     timer::{Timer, Timer0},
  |             ^^^^^  ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

[bjoernQ]

LGTM - Thanks!