This fixes the warning "compiler may not enforce left-to-right
evaluation order for call to" that is caused by Visual Studio if this is
compiled with a target of C++17.
Since both `std::string` and `std::filesystem::path` can be convertible
from `const char &[N]` (like, `parse("file.toml")`), after adding
`parse(std::filesystem::path)`, the overload resolution of
`parse("file.toml")` becomes ambiguous. By adding `parse(...)` that
exactly matches to `parse("file.toml")`, we can remove this ambiguity.
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.
