Change %p formatting to match glibc

[elipsitz]

The %p format is not defined by the standard, that is, it's implementation specific. However, many people are used to what glibc prints: 0x to emphasize that pointers are printed as hex, and a special "(nil)" string when a null pointer is provided, and lower-case letters for hex digits.

This fixes #57 and fixes #84.

[codecov-commenter]

Codecov Report

Merging #90 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #90   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          359       362    +3     
=========================================
+ Hits           359       362    +3     

Impacted Files	Coverage Δ
printf.c	100.00% <100.00%> (ø)

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[eyalroz]

Since this is non-standard, I suggest you have a preprocessor define controlling whether %p support is compiled-in or not. Also, what systems print %p the way the library does right now?