std: Speed up str::is_utf8

Use unchecked vec indexing since the vector bounds are checked by the
loop. Iterators are not easy to use in this case since we skip 1-4 bytes
each lap. This part of the commit speeds up is_utf8 for ASCII input.

Check codepoint ranges by checking the byte ranges manually instead of
computing a full decoding for multibyte encodings. This is easy to read
and corresponds to the UTF-8 syntax in the RFC.

No changes to what we accept. A comment notes that surrogate halves are
accepted.

Before:

	test str::bench::is_utf8_100_ascii ... bench: 165 ns/iter (+/- 3)
	test str::bench::is_utf8_100_multibyte ... bench: 218 ns/iter (+/- 5)

After:
	test str::bench::is_utf8_100_ascii ... bench: 130 ns/iter (+/- 1)
	test str::bench::is_utf8_100_multibyte ... bench: 156 ns/iter (+/- 3)
Rename IpAddr -> SocketAddr, extract IpAddr from SocketAddr

multicast functions now take IpAddr (without port), because they dont't
need port.

Uv* types renamed:
* UvIpAddr -> UvSocketAddr
* UvIpv4 -> UvIpv4SocketAddr
* UvIpv6 -> UvIpv6SocketAddr

"Socket address" is a common name for (ip-address, port) pair (e.g. in
sockaddr_in struct).
auto merge of #8237 : blake2-ppc/rust/faster-utf8, r=brson

Use unchecked vec indexing since the vector bounds are checked by the
loop. Iterators are not easy to use in this case since we skip 1-4 bytes
each lap. This part of the commit speeds up is_utf8 for ASCII input.

Check codepoint ranges by checking the byte ranges manually instead of
computing a full decoding for multibyte encodings. This is easy to read
and corresponds to the UTF-8 syntax in the RFC.

No changes to what we accept. A comment notes that surrogate halves are
accepted.

Before:

	test str::bench::is_utf8_100_ascii ... bench: 165 ns/iter (+/- 3)
	test str::bench::is_utf8_100_multibyte ... bench: 218 ns/iter (+/- 5)

After:
	test str::bench::is_utf8_100_ascii ... bench: 130 ns/iter (+/- 1)
	test str::bench::is_utf8_100_multibyte ... bench: 156 ns/iter (+/- 3)

An improvement upon the previous pull #8133
auto merge of #8243 : stepancheg/rust/ipv, r=brson

multicast functions now take IpAddr (without port), because they dont't
need port.

Uv* types renamed:
* UvIpAddr -> UvSocketAddr
* UvIpv4 -> UvIpv4SocketAddr
* UvIpv6 -> UvIpv6SocketAddr

"Socket address" is a common name for (ip-address, port) pair (e.g. in
sockaddr_in struct).

P. S. Are there any backward compatibility concerns? What is std::rt module, is it a part of public API?