Add hex.h to fmt.h

Add accidentally forgotten inclusion of "hex.h" into "fmt.h" to join
other format headers there.
Fix hex source position unpacking

Use '&' operator instead of '&&' in hidrd_hex_src_fmtpos to extract
position bitfield. This fixes hex source position formatting.
Stub unused format arguments

Add void string format specifiers to *printf invocations for unused
arguments. This silences Clang warnings.
Cast enum values to int when checking ranges

Cast collection types and usage pages to int when checking ranges as
they may take values besides defined enum members. This silences Clang
warnings.
Use proper argument type to item size validation

Use hidrd_item_pfx_bytes type (item size in bytes), instead of
hidrd_item_pfx_size (item size code) for passing item size in bytes to
corresponding validation function. This fixes Clang warnings. Fixes #6
"Verify comparisons between enum variables and hardcoded constants".
Cast *printf arguments to proper types in tests

Cast *printf arguments to types corresponding to format specifiers in
any_test and num_test to silence Clang warnings.