Ignore uniquely named benchmark files.
Update model-elimination.sml benchmark program and iteration count.

Eliminate uses of `CommandLine.arguments ()` from model-elimination
benchmark program.  When executed by the benchmark program, the
model-elimination program is executed with no command-line arguments.
However, when executed by the update-counts.sh script, the
model-elimination program is executed with an interation count
command-line argument, which led to a Fail exception and an erroneous
iteration count.
Merge pull request #80 from MatthewFluet/benchmark-infrastructure

Updates to benchmark infrastructure.

Ignore benchmark program executables.

Update model-elimination.sml benchmark program to eliminate uses of CommandLine.arguments () and fix iteration count.