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.