Current implementation (based on mypy behaviour):
- evaluate any fstring with expressions as `builtin.str`
- evaluate static fstrings (could be regular strings) as `Literal[..]`
  if it fits the max size or `LiteralString` (same as regular strings)

…sions

Resolves f-strings made entirely of literals (bool, int, str) at compile
time (not supported by mypy).

Example: 'f"{True}"' -> 'True'

… expressions

Implement the methods `Type::str` and `Type::repr` for some basic types
and use it when infering the type of an f-string expression.

Ensure we always infer sub-expressions in an f-string expression, even
when we're done figuring out its type.