Vectray cruising

A number of fixes and improvements.

• Added initial prototype of class Vectray. This is a compromise between a vector and an array. It provides the static allocation of an array up to a compile time limit, then internally changes to dynamic memory if the array storage is exhausted.
• Fixed edge case in svto_radix where overflow might not be detected.
• MemSpan now supports conversion from non-const to const.
• MemSpan can be automatically constructed from std::array.
• Added "lru_cache" example for a fast LRU cache using intrusive containers.
• Improved TextView::assign.
• Tweaked Unit Parser example.
• Hash support for IPAddr, IP4Addr, IP6Addr.
• Access to IPv6 address as MemSpan.