Add IPAddrBlock and ASIdentifiers extensions (RFC 3779)

[arckoor]

This PR implements two extensions required for working with secure routing. These extensions contain IP Addresses or AS numbers, as specified by RFC 3779.

Both encoding and decoding of AS numbers, single addresses, address prefixes and min-max ranges have been tested.

[coveralls]

coverage: 91.294% (+0.04%) from 91.258%
when pulling 81ac46d on arckoor:master
into 09a7a98 on randombit:master.

[randombit]

@arckoor There are some CI failures which are relevant. Stylistically for calls to min,max where the compiler can’t deduce T we prefer std::min<T> vs use of casts.

[randombit]

@arckoor Remaining CI issues look related to the visibility annotations (eg BOTAN_PUBLIC_API) probably something is not being exported as a symbol.

[arckoor]

@randombit We're currently chipping away at the fuzzers, but the visibility is also an issue. Previously linux/clang was failing, which I resolved by moving the definition of set_from_bytes to the .h file, the other errors could probably be solved by moving the rest there too, but that does not sound like a good solution to me. All of these errors are most likely related to the use of templates, though I don't know what the cleanest / recommended way to resolve them is.

[randombit]

@arckoor ok thanks. I’ll try to have a look over the weekend