- 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.
If unreleased feature is activated, zero-prefixes in an exponent part of
a floating point is allowed. If the flag TOML11_UNRELEASED_TOML_FEATURES
is turned on, we don't need to check whether there is a zero prefix in
the exponent part that is formatted by a standard library.
When non-table value is passed to the `operator<<`, it assumes that the
original C++ code looks like the following.
```cpp
std::cout << "key = " << v << std::endl;
```
In this case, the comment associated to `v` should be put just after
`v`, not before.
```toml
key = # comment <= bad
"value"
key = "value" # comment <= good
```
So, if `v` is not a table it would put comments just after the value.
