I've been using the new Criterion benchmarks and I noticed that they
take a _long_ time to build before the benchmark can run. Turns out
`cargo build` was building 3 separate benchmarking binaries with most of
Nu's functionality in each one.
As a simple temporary fix, I've moved all the benchmarks into a single
file so that we only build 1 binary.
### Future work
Would be nice to split the unrelated benchmarks out into modules, but
when I did that a separate binary still got built for each one. I
suspect Criterion's macros are doing something funny with module or file
names. I've left a FIXME in the code to investigate this further.
