Don't export Tny_swapBytes*.

Generic libraries should not expose anything about byte-ordering
differences via their headers. By making Tny_swapBytes static,
optimizing compilers can aggressively inline these endian-specific
functions into the library itself to both reduce size and improve
performance.
Don't declare tnyHostOrder in the API header.

Declaring a global object in a header is dangerous, as two
translation units that are compiled separately and both include
the tny header file can result in multiple instances of the
tnyHostOrder union.
Replace stdio.h with stddef.h in API header.
Encourage const-correctness.

Use const to assist optimizing compilers and to help reduce
programming errors by labeling functions which should not, and do
not, change Tny data members as const pointers.