Updates \map command (\function now obsolete)

The \map command now takes the syntax
	\map[f]{X}[hook]{Y}[x][y]
where:
  f	function
  X	source
  hook	arrow modifier (e.g., hook for injective maps)
  Y	target
  x	element in source
  y	image of x
Changes all maps to reflect the new \map syntax
Adds small tweaks, mainly improves typography
Adds two paragraphs on weak/hard Lefschetz theorems