Make preemption knobs configurable; fix unused-variable warnings

Add `Config::alloc_interval()` and `Config::timeslice_cycles()` so
callers can tune preemption sensitivity at runtime. The values flow
through `RuntimeInner` and are written into per-scheduler-thread locals
via a new `configure_preempt()` call at thread startup, keeping the hot
path free of cross-thread coherency traffic.

Fix unused-variable warnings in channel.rs by inlining `current_pid()`
directly into `te!` macro arguments — since the no-op macro arm never
evaluates its argument, no binding is needed at the call site.

Clean up a handful of dead imports exposed by the refactor.

Cargo.toml

@@ -12,7 +12,7 @@ libc = "0.2"
 
 [dev-dependencies]
 libc = "0.2"
-tokio = { version = "1", features = ["rt", "rt-multi-thread", "macros", "sync"] }
+tokio = { version = "1", features = ["rt", "rt-multi-thread", "macros", "sync", "time"] }
 
 [profile.dev]
 panic = "unwind"