Use the internal slice api

src/core/ext/client_channel/client_channel_plugin.c

src/core/ext/client_channel/http_connect_handshaker.c

src/core/ext/client_channel/parse_address.c

src/core/ext/client_channel/resolver_registry.c

src/core/ext/client_channel/resolver_registry.h

src/core/ext/client_channel/subchannel.c

src/core/ext/client_channel/subchannel.h

src/core/ext/client_channel/uri_parser.c

src/core/ext/client_channel/uri_parser.h

src/core/ext/lb_policy/grpclb/grpclb.c

src/core/ext/transport/chttp2/client/chttp2_connector.c

src/core/lib/iomgr/tcp_client_posix.c

test/core/client_channel/parse_address_test.c

...client_channel/resolvers/dns_resolver_connectivity_test.c

test/core/client_channel/resolvers/dns_resolver_test.c

test/core/client_channel/resolvers/sockaddr_resolver_test.c

test/core/client_channel/uri_fuzzer_test.c

test/core/client_channel/uri_parser_test.c