Set a limit on WHNF computation depth

[fizruk]

• Add a limit on how many nested WHNF calls are allowed
• Default limit is 50
• The limit can be changed in the Rzk code, for example, to set the new depth (starting at a certain point in the formalization) to 100, simply add #set-option "max-whnf-depth" = "100" at the top-level. To reset the depth to default, do #unset-option "max-whnf-depth"