Remove hacky `StringRef` cache

[raviqqe]

Problem

• The implicit global StringRef cache grows forever.
• We can never clean them safely.
• That makes the use of Melior impossible in compilers that share states in memory among compilation of modules.

Solution

• Should the MLIR C API accept non-null-terminated strings?
• Should we move this into a Context type making it a non-transparent type?

https://github.com/raviqqe/melior/blob/2118d871f7729714fd5e34986e45ba6513736ed7/src/string_ref.rs#L14