Remove allocations due to sorting keys in NameLookup

[kevin-montrose]

Visible changes:

• The MemoryPool<char> on an Options will see more use during construction of NameLookup when deserializing dynamic rows (first row read only)

Internal changes:

• Closes #4 - there are no more mandatory heap allocations when creating a NameLookup
• NameLookup construction now expects the same collection to provide names and indexes, rather than the two used before
• NameLookup now expects column names to be stored in an array, whereas before it could run over any IEnumerable<string>