Bindings to nng.

Version corresponds to C library

• http - Builds with http code ON
• websocket - Builds with websocket code ON. Enables http alongside
• tls - Builds with TLS code ON
• stats - Builds with statistics collection. See this for details.

When tls feature is enabled this crate compiles mbedtls 2.28.8 to bundle it together with nng

Specify environment variable ANDROID_NDK_HOME which points too root of NDK installation where to look for toolchain file