tex-l3kernel{,-doc}: update to 2020.57669

## [2021-02-06]

### Changed
- Use new (internal) interface for kerns

## [2021-02-02]

### Added
- `\c_zero_str`

## [2021-01-09]

### Added
- `\keyval_parse:nnn`

### Changed
- `\keyval_parse:NNn` is set equal to `\keyval_parse:nnn`

### Fixed
- Handling of encoding-specfic commands in `\text_purify:n`

(markd)