improve the use case when function under $NAME isn't found

[petr-tik]

Problem description

Running cargo asm builds the binary with --release by default. The compiler is smart enough to optimise some functions away. Those functions then won't be found when running cargo asm crate::function::was_optimised_away

Improvement suggestion

• When function isn't found, check the build settings again. If the binary was built with opt-level>1 or in --release mode, print a hint to stdout along the lines of Building with opt-level=$OPT_LEVER might optimise some functions away. Consider changing opt-level=0 in Cargo.toml
• Add documentation/explanation to the Reame

[gnzlbg]

This won't help for functions marked #[inline] or #[inline(always)]. This won't also help for generics that are not monomorphized, nor for unused items (e.g. non-exported non-used functions), so I don't think this is worth the trouble.

The error message notes should be improved in the spirit of what is discussed in #23 to make it clear in which cases can the compiler actually emit machine code or llvm-ir for a function.