perf(dict): Bypass vars when possible

[epage]

Variant support slows us down by 10-50$. I assume most people will run
with en and so most of this overhead is to waste. So instead of
merging vars with dict, let's instead get a quick win by just skipping
vars when we don't need to. If the assumptions behind this change over
time or if there is need for speeding up a specific locale, we can
re-address this.

Before:

check_file/Typos/code   time:   [35.860 us 36.021 us 36.187 us]
                        thrpt:  [8.0117 MiB/s 8.0486 MiB/s 8.0846 MiB/s]
check_file/Typos/corpus time:   [26.966 ms 27.215 ms 27.521 ms]
                        thrpt:  [21.127 MiB/s 21.365 MiB/s 21.562 MiB/s]

After:

check_file/Typos/code   time:   [33.837 us 33.928 us 34.031 us]
                        thrpt:  [8.5191 MiB/s 8.5452 MiB/s 8.5680 MiB/s]
check_file/Typos/corpus time:   [17.521 ms 17.620 ms 17.730 ms]
                        thrpt:  [32.794 MiB/s 32.999 MiB/s 33.184 MiB/s]

This puts us inline with --no-default-features --features dict

Fixes #253