Qualified names for both `IntrinsicSym` and `NormalSym`

Currently IntrinsicSym serialises with the short name (e.g. black_box) and NormalSym serialises with the mangled name (e.g. _ZN4core9panicking19assert_failed_inner17h1d286061ca0adfe7E). Both have upsides and downsides:

• short name is concise but ambiguous (which black_box? Hard to tell just by reading)
• mangled name is unambiguous but too verbose

#85 was going to use the mangled name for intrinsics, but this would not fix the verbosity problem.

A middle ground for both would be a qualified name

• _ZN4core9panicking19assert_failed_inner17h1d286061ca0adfe7E -> core::panicking::assert_failed_inner
• black_box -> core::intrinsics::black_box or core::hint::black_box

This comment for some more context