uint32_t/size_t review: __wt_raw_to_hex_mem() should take a size_t
length, not a uint32_t.

While I was in the area, the macro FILL_HEX had some minor fluff: it
declared variables in the function's namespace, some macro arguments
were used without beingt parenthesized, the test for destination buffer
length could potentially wrap around if the destination buffer length
was too short.  None of these are bugs in the existing code, but there's
no reason to leave it that way either.  I rewrote FILL_HEX as an inline
function instead, and fixed some comments, there's no "worst case" for
this function, it converts N bytes to a nul-terminated (N * 2 + 1)
string.