debug: get values of more types: uintptr, bool, float, complex, pointer, array, and struct.

Also handles the DWARF types TypedefType, CharType, and UcharType.  In
program.go, adds more implementations of Var: Pointer, Array, and
Struct.  In proxyrpc.go, adds gob registration for those types in an
init() function.

Change-Id: Ie3a6c3a7ef670310f2be6314f27f5c0ddc92a661
Reviewed-on: https://go-review.googlesource.com/10562
Reviewed-by: Rob Pike <r@golang.org>