Improve startup time (regression)

[sharkdp]

The new type checker from #443 introduced a severe regression in startup times:

hyperfine --warmup 3 --reference "./numbat-master -e '1+2'" "./numbat-1.12 -e '1+2'" --export-markdown -

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./numbat-master -e '1+2'	78.9 ± 1.2	77.1	83.8	2.28 ± 0.09
./numbat-1.12 -e '1+2'	34.5 ± 1.2	32.4	42.5	1.00

We should look into potential measures to improve this again.

If the constraint checker is the culprit, we might want to look into de-duplication of constraints before attempting to solve them. But it might be something else entirely (e.g. the mutation/traversal/cloning of ASTs during elaboration)

[sharkdp]

Thanks to the changes in #543 by @GuerricChupin, we've moved a significant step in the right direction:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./numbat-master -e '1+2'	59.9 ± 1.2	57.9	64.3	1.78 ± 0.08
./numbat-1.12 -e '1+2'	33.7 ± 1.3	32.6	44.8	1.00

[sharkdp]

Thanks to the changes in #537 by @irevoire (which are not related to the type checker, but still), we've moved another significant step in the right direction:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./numbat-master -e '1+2'	47.0 ± 0.9	45.5	50.3	1.37 ± 0.04
./numbat-1.12 -e '1+2'	34.2 ± 0.9	32.2	36.7	1.00