This commit adds set of generic binary search related functions in
OpenThread core under `BinarySearch` module. The binary search module
replaces and enhances the existing `Lookup` table module (which was
limited to search based on strings and mainly used by CLI to map from
command string to a handler function).
This commit also adds a new class `Stringfy` in `string.hpp` to help
convert from a set of `uint16_t` values (e.g., a non-sequential
`enum`) to strings using binary search in a table. The new methods are
used for converting `Coap::Code` and `IpProto` enumerations to
corresponding strings.
Abtin Keshavarzian