To allow the following toml file, we need to replace the region after
the more precise region is found.
```toml
[a.b.c]
d = 42
[a]
e = 2.71
```
If the precise region (here, [a]) is found, the region of `a` should be
`[a]`, not `[a.b.c]`. After `[a]` is defined, toml does not allow to
write `[a]` twice. To check it, we need to replace the region of values
to the precise one.
The function was needed to copy region information from value to value,
for a useful error message. Because of the last few commits, the region
information about keys are passed to insert_nested_keys that requires
the function which is removed. And it turned out that the function is no
longer required. It is originally a workaround, so I removed it.
it is convenient to have get_region function that can access region_info
in toml::value. get_region is placed in toml::detail and made friend of
toml::value because I don't want to make toml::value::region_info public
and keep it internal use only.
