Resolve many bslx warnings (#2241)
Resolve many bslx warnings (#2241)

It turns out that over 1000 lines of warnings in one test driver
are swamping the ability to see other warnings in 'bslx'.  This
patch simply applies an update to use 'size_t' where appropriate
(long outstanding since 'vector::size()' became unsigned), after
comparing with other (non-test) 'bslx' output streams that this
implementation should be valid.
che2/bde,che2/bde,bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde