Fix "first" sort for inline tables

[vfazio]

Previously, sorting inline tables would throw a ValueError if the key in
the table had trailing spaces.

For example:

  [a]
  b = {c=1, d = 3}

Sorting a.b with "d" first would fail.

Removing the space would "pass":

  [a]
  b = {c=1, d=3}

However, when the table was written back out, rerunning toml-sort would
fail because the keys are written with surrounding spaces:

  [a]
  b = {c = 1, d = 3}

When commit 9900c71 was introduced, the value used for the index
lookup in the list of "first" keys changed to be Key.as_string().

The intent was probably to pass type checks as the list of keys are
strings and mypy complains when doing an index lookup that doesn't match
the typing of the list elements (str vs Key).

However, as_string() uses the "original" value of the key which
includes spaces and is subtlely different than the check performed
on the previous line that checked if the value was in the list.

Both the in keyword and the list.index method use the equality
operation to evaluate if a value is in a list.

The Key object defines the __eq__ method that compares the internal
key value (which is stripped) to the other value. This is why the in
lookup succeeds but the index lookup fails.

Now, the lookup logic is simplified.

Instead of potentially searching the list of "first" keys twice, once
via in and once via list.index, iterate the list via enumerate to
keep track of the index and perform an explicit equality check against
Key.key which appeases mypy.

[pappasam]

🚀 thanks, this lgtm!