* Performance improvement of `scan_line_break`.
* Rename `scan_line_break` to `yaml_1_1_scan_line_break`.
* Add `yaml_1_2_scan_line_break`.
* Add better comments.
* Add a TODO comment about possible bugs.

* Bug fix.
* Change to use `yaml_1_1_` prefix for YAML 1.1's `forwardchars!`.
* Add `yaml_1_2_forwardchars!` for YAML 1.2's `forwardchars!`.
* Add some helper functions for `forwardchars!`:
  * `forwardchar_skip!`
  * `forwardchar_nobreak!`
  * `forwardchar_breakline!`

Here we use abstract type & subtyping because it's common traits pattern
in Julia.
We do not need to export these objects because we can use strings for
versions in user-facing functions like:

```julia

function load(str::AbstractString; version::YAMLVersion)
    # ...
end

function load(str::AbstractString; version::AbstractString)
    version == "1.1" ? load(str, version=YAMLV1_1()) :
    version == "1.2" ? load(str, version=YAMLV1_2()) :
    throw(ErrorException())
end

load(str, version="1.1")
```

fixed.

…o be sorted.

* Sort out functions.
* Add `b-char` for YAML 1.2.
* Add comments to `forwardchars!`.