Add a utility crate for quick evaluation

Introduce a simple binary that can run arbitrary input against any of
the available implementations (musl, MPFR, our libm). This provides an
easy way to check results, or run specific cases against a debugger.

Examples:

    $ cargo run -p util -- eval libm pow 1.6 2.4
    3.089498284311124
    $ cargo run -p util -- eval mpfr pow 1.6 2.4
    3.089498284311124
    $ cargo run -p util -- eval musl tgamma 1.2344597839132
    0.9097442657960874
    $ cargo run -p util -- eval mpfr tgamma 1.2344597839132
    0.9097442657960874
    $ cargo run -p util -- eval libm tgamma 1.2344597839132
    0.9097442657960871
    $ cargo run -p util -- eval musl sincos 3.1415926535
    (8.979318433952318e-11, -1.0)

library/compiler-builtins/libm/crates/libm-test/src/mpfloat.rs

@@ -471,6 +471,32 @@ impl MpOp for crate::op::lgammaf_r::Routine {
     }
 }
 
+/* stub implementations so we don't need to special case them */
+
+impl MpOp for crate::op::nextafter::Routine {
+    type MpTy = MpFloat;
+
+    fn new_mp() -> Self::MpTy {
+        unimplemented!("nextafter does not yet have a MPFR operation");
+    }
+
+    fn run(_this: &mut Self::MpTy, _input: Self::RustArgs) -> Self::RustRet {
+        unimplemented!("nextafter does not yet have a MPFR operation");
+    }
+}
+
+impl MpOp for crate::op::nextafterf::Routine {
+    type MpTy = MpFloat;
+
+    fn new_mp() -> Self::MpTy {
+        unimplemented!("nextafter does not yet have a MPFR operation");
+    }
+
+    fn run(_this: &mut Self::MpTy, _input: Self::RustArgs) -> Self::RustRet {
+        unimplemented!("nextafter does not yet have a MPFR operation");
+    }
+}
+
 /// `rug` does not provide `remquo` so this exposes `mpfr_remquo`. See rug#76.
 fn mpfr_remquo(r: &mut MpFloat, x: &MpFloat, y: &MpFloat, round: Round) -> (Ordering, c_long) {
     let r = r.as_raw_mut();