Add a few more sources to CONV_SRCS.
Turn off -Werror by default.

It inteferes with development activities by breaking your build in the
middle of a refactor.  Instead, let's enable -Werror on the Travis CI
builds via a TRAVIS_CI_BUILD option.
Refactor: Redefine Map(T) on top of the more generic VMap(T, U)

The VMap(T, U) can receive the value type as argument, for cases where one
to store scalar values