If building release mode, remove debug code. Now, the `debug_print!`
macro resolves to nothing in release build.
In debug build, don't check the environment for each print. This has a
huge overhead. The environment is only checked once and next checks are
made against a simple boolean value.
This is a huge commit that cannot easily be broken down as it contains
fixes for the next ignored test in the suite which, one fixed, broke
tests that used to pass and were only then fixed.
There is also a substantial amount of comments that were added,
especially around `SimpleKey`. Minor improvements around the code were
added and I did not bother making a separate commit for them.
Overall, that commit fixes 7 tests from the matrix that were related to
the handling of simple keys.
Change scanner's complex test:
```diff
- *coffee:
+ *coffee :
amount: 4
- *cookies:
+ *cookies :
amount: 4
```
According to https://play.yaml.io/main/parser, this example was invalid
in the first place. Adding a space makes it so that the colon is not
part of the alias name.
Also fix colons not being able to be part of anchors.
YAML 1.2 has special handling of indicators to be compatible with JSON.
The following is equivalent to `{"a": "b"}` (note, no space after `:`):
{"a":b}
But without the quoted key, a space is required. So the `:` here is part
of the plain scalar:
{a:b} # == {"a:b"}
A plain scalar can also start with a `:` as long as it's followed by
"safe" characters:
{a: :b} # == {"a": ":b"}
(Fixes#118)
