startuptime: rename prev_timeval to g_prev_time
startuptime: remote tv_ prefixes

They no longer refer to timeval's (and hungarian notation is not nice
anyway).
startuptime: document time_start
startuptime: use scientific notation for consts
garray: add necessary includes

The upcoming decoupling of vim.h and globals.h needs this to compile.
globals.h: decouple vim.h and globals.h

Allow globals.h to be included without including vim.h. Another small piece
of the puzzle of dismantling vim.h.

Moving some extra `#define`'s to globals.h is no better than having them in
vim.h. We should, in a later PR, move them to the file where they belong or
to a separate `constants.h` or something.
startuptime: always enable startuptime

Removes the STARTUPTIME define.