os: add fast path to mkdir_all

[ttytm]

mkdir_all is used at many places to ensure a destination directory exists - even when it's likely that it already does. Often it is not wrapped in an os.exists(<dst_dir>) condition. When it is, it should imho not be necessary to add the condition.

This PR increases the performance of mkdir_all when it is called outside a conditional block like mentioned above and the destition dir is already present. It does it by using an early return fast path. The performance increase is more than 300%.

It's the way go does it:
https://github.com/golang/go/blob/dd88f23a2006307f835f42063b5168ec56c2c428/src/os/path.go#L18-L24

The error is kept the same that would be thrown further down the function.

🤖 Generated by Copilot at e11c57e

Improve os.create_folder function by checking output path validity. Prevent file overwriting and nested folder creation.

🤖 Generated by Copilot at e11c57e

• Add a check for the output path existence and type before creating a folder (link). This prevents overwriting an existing file or creating a nested folder in os.create_folder.

[spytheman]

It needs a test for the new error too, in vlib/os/file_test.v or in vlib/os/os_test.c.v .

[spytheman]

The CI failure needs a rebase over master. It is not related to this PR.