std::forward_list does not have push_back, insert, or emplace but
push_front, insert_after, and emplace_after. We need to distinguish it
from other continers.
The top-level table has its region at the first character of the file.
That means that, in the case when a key is not found in the top-level
table, the error message points to the first character. If the file has
its first table at the first line, the error message would be like this.
```console
[error] key "a" not found
--> example.toml
|
1 | [table]
| ^------ in this table
```
It actually points to the top-level table at the first character,
not `[table]`. But it is too confusing. To avoid the confusion, the
error message should explicitly say "key not found in the top-level
table".
- if a basic string contains any double quote, make it multiline.
- because 1 or 2 consecutive "s do not require escape sequence in it.
- if a basic string will be sufficiently long, make it multiline.
- if 3 consecutive "s appeared, insert backslash to break it down.
In literal strings, only the first newline will be trimmed.
```toml
str = '''
The first newline will be trimmed.'''
```
The previous code always adds this first-newline, but after this commit
it checks the length of the string and adds newline if the string is
sufficiently long.
According to toml-lang/toml:36d3091b3 "Clarify that inline tables are
immutable", check if it adds key-value pair to an inline table.
This is one of the unreleased (after-0.5.0) toml feature. But this is
marked as "Clarify", so TOML-lang intended that inline tables are
immutable in all version.
Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
of local_date and local_time independently, the conversion fails if
it is the day when DST begins or ends. Since local_date considers the
time is 00:00 A.M. and local_time does not consider DST because it
does not have any date information. We need to consider both date and
time information at the same time to convert it correctly.
