I'm not sure what IS "all over there" (in all that different locations instead in one convenient place) but what I'd like to see...

Core and stdlib.h:
Casting (signed-unsigned, char-float, int-float, float-int, float-char, atoi(), atof(), some more I've forgotten.)
all of int(16bit) operators like +,-,*,/, ++, --, %, &, |, ~, >>, <<, *= etc. Same about float.
Relations: >, <, >=, <=, <>, ==
rand, srand, maybe more.
BCD casts.

string.h:
mostly all.

math.h
all.

stdio.h, io.h: All that makes sense in assembler.

regex.h - some basic regex support.

termios.h - all settings of serial/paralell ports.

time.h - quite a lot...

Most of the varioust useful routines found in the "Code Library".

Certainly much more, but right now it's too early in the morning and thinking hurts.