Juri

Closes #15775

`pivot()` doesn't distinguish between `count` and `len`

3

### Checks - [X] I have checked that this issue has not already been reported. - [X] I have confirmed this bug exists on the [latest version](https://pypi.org/project/polars/) of Polars. ###...

`separator` parameter in `pivot()` has no effect

5

### Checks - [X] I have checked that this issue has not already been reported. - [X] I have confirmed this bug exists on the [latest version](https://pypi.org/project/polars/) of Polars. ###...

Closes #15775

`use_columns` silently skips empty columns at the beginning

6

Given the following spreadsheet with an empty first column:  Loading the sheet with e.g. `use_columns='B:C'` (or `use_columns=[1, 2]`) actually selects columns C and D: ```python read_excel('sheet.xlsx').load_sheet(0, use_columns='B:C').to_polars() shape: (3,...