refactor (datatypes): add from_str constructor

[OutSquareCapital]

This PR add a new DatatType.from_str() classmethod constructor, and modify call sites to use it when we know a str will be used.

This brings the following benefit:

• Performance improvement, since we now avoid a redundant isinstance check
• build is more readable
• A few call sites were using build when DType.into_expr() could be directly used instead. Now fixed
• Use Self for return types and cls instead of DataType (class name agnostic).

Note

I still hesitate between from_str and parse for the method name.

[OutSquareCapital]

Here we were passing a dialect, even tough kind is statically known to be a DType instance. So this parameter had no effect at runtime. better to directly use DType.into_expr()

[georgesittas]

Performance improvement, since we now avoid a redundant isinstance check

@OutSquareCapital did you benchmark this? Can you provide some actual numbers to understand the impact of this change? Cover both paths where the inputs are strings and paths where inputs are non-strings to measure the impact.

[georgesittas]

Why was this section until L373 refactored?