File-level inline assembly blocks

Just like free functions, inline assembly blocks could be allowed at file level. This would make it possible to write helper functions in yul that can be called from any inline assembly block. Maybe we could even import these yul functions using import statements.

Start by studying how free functions and inline assembly blocks are currently represented and resolved in the Solidity compiler. Then investigate how file-level Yul functions would interact with calls from separate inline assembly blocks and whether import statements should be supported. Done means the language design, scope, and required implementation and tests are agreed.

Written by the indexing model from the issue text.