Add bounded memory-safety harnesses for core::num::flt2dec (challenge #28)

.gitignore

@@ -20,6 +20,7 @@ Session.vim
 /book/
 /build/
 /kani_build/
+/cbmc_build/
 /target
 library/target
 scripts/autoharness_analyzer/target/

library/core/src/lib.rs

@@ -64,6 +64,8 @@
 #![no_core]
 #![rustc_coherence_is_core]
 #![rustc_preserve_ub_checks]
+// Composed Kani stubs require more macro expansion depth.
+#![cfg_attr(kani, recursion_limit = "512")]
 //
 // Lints:
 #![deny(rust_2021_incompatible_or_patterns)]