Validate list of TomlItems and convert to TOML if not validation errors are found.

Error that happens during validating TOML which is already syntactically correct. For the list of all possible validation errors and their explanation, see the following issue on GitHub:

• https://github.com/kowainik/tomland/issues/5

This function takes flat list of TomlItems and groups it into list of Trees by putting all corresponding items inside tables and table arrays. It doesn't perform any validation, just groups items according to prefixes of their keys. So, for example, if you have the following keys as flat list:

aaa              # ordinary key
aaa.bbb          # ordinary key
[foo]            # table nam
foo.bar
foo.baz
[xxx]            # table name
[xxx.yyy]        # table name
zzz

the following tree structure will be created:

aaa
aaa.bbb
[foo]
├──── foo.bar
└──── foo.baz
[xxx]
└──── [yyy]
      └──── zzz

This function groups list of TOML items into Forest and returns list of items that are not children of specified parent.

Invariant: When this function is called with Nothing, second element in the result tuple should be empty list.

Construct TOML from the Forest of TomlItem and performing validation of TOML at the same time.