Unbreak basic pack/unpack functions

Unfortunately I don't understand why these need explicit module
references.
Filled out README/DESCRIPTION
merge() is now in Base
libLLVM can use the simplified ccall()
StrPack: fixed array support

This commit is the sum of a near-complete rewrite of StrPack required to
properly support fixed arrays and strings. The documentation is still
very incomplete, but there are some simple tests, and the module should
be usable at this stage.