MD5 Builder: Rework for unqiue_ptr instead of malloc for internal dynamic buffers.

[lrodorigo]

In order to guarantee exception-safety I used unique_ptr to uint8_t[], with std::nothrow decorator.
I cannot mark the functions as no-except because I don't know if the called functions (especially the Stream implementations) are no-except.

I also fixed the const-correctness of const member functions, such as toString() and getters.

I executed the host tests, I still need to run tests on the ESP8266, but this is the first time that I contribute with you, so I have to study how to setup and run your tests suite on my device.

[earlephilhower]

Thanks, @lrodorigo !

[lrodorigo]

@earlephilhower I'm so happy to cooperate with you, in this important project. If you think that I'm appropriate, please let me know if I can cooperate with you in a more stable way and what I can do now.

Regards

[devyte]

@lrodorigo if you would like to help move the core forward, please msg me in our gitter channel gitter.im/esp8266/.