Move the start of TR_allocVector to the start of allocVector

Change TR_allocVector to cover the entire allocVector function
including the special case for length 1 vectors and the preprocessing
that calculates the allocation length. The original placement was
chosen to keep the time result compatible with the original timeR
implementation, but moving it simplifies the documentation.
Attempt to estimate the timing overhead

Add an estimate for the timing overhead to the output file. It is
calculated by starting/stopping a timer 10 or 1000 times before and
after R itself runs. The average number of measurement units per timer
start are shown as "OverheadEstimate" in the output file. The values
are different because the 1000 iteration measurement profits a lot
more from the CPU cache than the 10 iteration measurement, so they are
shown as a value range.