Revert "Fix minor typos."

This reverts commit 5ff2696bb18d571546960caec0212ceaae056cff.
Revert "Split benchmark models in separate files to ease performance analysis."

This reverts commit 11acadf014b0f783ca1cc8f461989025773cb2ee.
Revert "Compile only one small model."

This reverts commit 269cbd039ce91ed0bf153cf3be8452c4d33cd103.
add single-model benchmarks

which are easier to use when tracking regressions.

the multi-model benchmark is kept because it is handy for quick checks
and also helps detecting symbol collisions.

This commit also removes the BENCHMARK macro, anymore since the per-model
namespace was abandoned.