Rollup merge of #111009 - scottmcm:ascii-char, r=BurntSushi

Add `ascii::Char` (ACP#179)

ACP second: https://github.com/rust-lang/libs-team/issues/179#issuecomment-1527900570
New tracking issue: https://github.com/rust-lang/rust/issues/110998

For now this is an `enum` as `@kupiakos` [suggested](https://github.com/rust-lang/libs-team/issues/179#issuecomment-1527959724), with the variants under a different feature flag.

There's lots more things that could be added here, and place for further doc updates, but this seems like a plausible starting point PR.

I've gone through and put an `as_ascii` next to every `is_ascii`: on `u8`, `char`, `[u8]`, and `str`.

As a demonstration, made a commit updating some formatting code to use this: https://github.com/scottmcm/rust/commit/ascii-char-in-fmt (I don't want to include that in this PR, though, because that brings in perf questions that don't exist if this is just adding new unstable APIs.)

tests/codegen/ascii-char.rs

@@ -0,0 +1,37 @@
+// compile-flags: -C opt-level=1
+// ignore-debug (the extra assertions get in the way)
+
+#![crate_type = "lib"]
+#![feature(ascii_char)]
+
+use std::ascii::Char as AsciiChar;
+
+// CHECK-LABEL: i8 @unwrap_digit_from_remainder(i32
+#[no_mangle]
+pub fn unwrap_digit_from_remainder(v: u32) -> AsciiChar {
+    // CHECK-NOT: icmp
+    // CHECK-NOT: panic
+
+    // CHECK: %[[R:.+]] = urem i32 %v, 10
+    // CHECK-NEXT: %[[T:.+]] = trunc i32 %[[R]] to i8
+    // CHECK-NEXT: %[[D:.+]] = or i8 %[[T]], 48
+    // CHECK-NEXT: ret i8 %[[D]]
+
+    // CHECK-NOT: icmp
+    // CHECK-NOT: panic
+    AsciiChar::digit((v % 10) as u8).unwrap()
+}
+
+// CHECK-LABEL: i8 @unwrap_from_masked(i8
+#[no_mangle]
+pub fn unwrap_from_masked(b: u8) -> AsciiChar {
+    // CHECK-NOT: icmp
+    // CHECK-NOT: panic
+
+    // CHECK: %[[M:.+]] = and i8 %b, 127
+    // CHECK-NEXT: ret i8 %[[M]]
+
+    // CHECK-NOT: icmp
+    // CHECK-NOT: panic
+    AsciiChar::from_u8(b & 0x7f).unwrap()
+}