struct SelectDelimiter

A traits-like metafunction for selecting the default Delimiter object type

for a particular Delimiter type. The base case simply exposes type Delimiter

itself as the delimiter's Type. However, there are specializations for

string-like objects that map them to the ByString delimiter object.

This allows functions like absl::StrSplit() and absl::MaxSplits() to accept

string-like objects (e.g., ',') as delimiter arguments but they will be

treated as if a ByString delimiter was given.