Print.print optimization. Closes #1760
Optimized Print::print(String&) (part 2)
Add Print::write(const char *, size_t)

The new function just calls Print::write(const uint8_t *, size_t), but
this allows writing out a buffer of chars (without having to learn about
casts).
Add uint8_t* versions of methods in Stream

The new functions just call their char* equivalents, but this allows
reading bytes into a buffer of uint8_t as well as chars.
Remove unneeded casts in Print::write(const String&)

Now that Print::write(const char*) is also available, these casts are no
longer needed.
sam: Optimized delayMicroseconds() (Rob Tillaart)

See #1121
Removed unused stuff from WiFi library.
Removed unused "socket.h" interface from WiFi library.

(that was probably an heritage from Ethernet lib)
Moved vanilla parts of WiFi library into common source folder.
WiFi librari: SPI delay is computed based on F_CPU constant
WiFi library is now vanilla
Merge pull request #1762 from matthijskooijman/ide-1.5.x-write-char

Support both char* and uint8* in Stream and Print