VM: Fix Windows VM build following type renaming

Making cell be a typedef of uintptr_t broke a spot assigning
cell* was unsigned long*. Added an explicit cast.
VM: Fix Windows 64-bit build on Visual Studio 2012

The UNW_FLAG_EHANDLER #define is now visible in VS2012, causing
a conflict with the definition in os-windows-x86.64.cpp. Added
a #ifndef to only include the definition if it hasn't already
been defined.
formatting: adding vprintf by request.
combinators.extras: adding keepd.
furnace.auth.providers.db: make test db name use cpu name.
combinators.extras: adding plox by request.
math.matrices: faster square-rows and square-cols.
math.matrices: simpler square-rows.
combinators.smart: Clarify stack effect for cleave>array.
math.combinatorics: adding a k-permutations virtual sequence.
math.combinatorics: use cond in k-permutations.
io.pipes: If dispose on in>> throws, then out>> leaks.
io.launcher: Fix two potential bugs.
1) If first (pipe) succeeds but second one fails, first pipe leaks.
2) out>> dispose fails, then in>> dispose is never called and leaks.
io.launcher: Fix bug -- if dispose fails, still call dispose on the other
resource.
sequences: change filter-as to allocate length of seq not exemplar.
destructors: more efficient dispose-each (not using make).
twitter: adding verify-credentials.
sequences.extras: change filter-index to use length of seq not exemplar.
sequences.extras: similarly improve map-filter and filter-map.
sequences.extras: adding index-selector.