Address style, robustness, and usability nits in the common endianess
conversion helpers in the libsigrok-internal.h header file. Rephrase
preprocessor macros as static inline C language functions to eliminate
side effects, and improve data type safety. Provide macros under the
previous names for backwards compatibility, so that call sites can
migrate to the routines at their discretion (or not at all).
Performance is not affected. Inline routines are identically accessible
to compiler optimizers as preprocessor macros with their text expansion
are. Resulting machine code should be the same.
Introduce variants which also increment the read or write position in
the byte stream after data transfer. This reduces more redundancy at
call sites.