Rename `LeftTrim` and `RightTrim` to `TrimLeft` and `TrimRight` accordingly

[arogachev]

@samdark's suggestion. Extracted from #90 (comment).

[samdark]

We can rename, introduce fallback classes and mark them as deprecated.

[vjik]

In PHP, similar functions are called ltrim() and rtrim(). May be keep as is for consistency?