Godot core needs MD5/SHA256/AES/Base64 which used to be provided by
separate libraries.
Since we bundle mbedtls in most cases, and we can easily only include
the needed sources if we so desire, let's use it.
To simplify library changes in the future, and better isolate header
dependencies all functions have been wrapped around inside a class in
`core/math/crypto_base.h`.
If the mbedtls module is disabled, we only bundle the needed source
files independently of the `builtin_mbedtls` option.
If the module is enabled, the `builtin_mbedtls` option works as usual.
Also remove some unused headers from StreamPeerMbedTLS which were
causing build issues.
Fixes the following GCC 5 warnings and actual bugs:
```
drivers/unix/net_socket_posix.cpp:562:28: warning: comparison between 'enum IP::Type' and 'enum NetSocket::Type' [-Wenum-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: comparison of constant '17' with boolean expression is always true [-Wbool-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: comparison of constant '6' with boolean expression is always false [-Wbool-compare]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/mbedtls/stream_peer_mbed_tls.cpp:286:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
modules/mbedtls/stream_peer_mbed_tls.cpp:313:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
```
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
