Implement stricter and checked multilanguage

[dodomorandi]

This PR is a bit messy, because we now have a fallible element deep in the tree, but in order to keep good ergonomics for the user it is necessary to fail only on build. This means that we need to keep both the unchecked and the checked versions of many structs around.

[codecov-commenter]

Codecov Report

Merging #70 (8c48181) into master (ce6a0fd) will increase coverage by 0.19%. The diff coverage is 99.64%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   96.67%   96.86%   +0.19%     
==========================================
  Files           8        8              
  Lines       10823    11625     +802     
==========================================
+ Hits        10463    11261     +798     
- Misses        360      364       +4     

Impacted Files	Coverage Δ
src/builder/data_schema.rs	97.27% <99.40%> (+0.36%)	:arrow_up:
src/builder.rs	97.82% <100.00%> (+0.05%)	:arrow_up:
src/builder/affordance.rs	96.12% <100.00%> (+0.19%)	:arrow_up:
src/builder/human_readable_info.rs	100.00% <100.00%> (ø)
src/thing.rs	93.67% <100.00%> (+0.12%)	:arrow_up:

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[lu-zero]

Digging the codecov report:

• impl<DS, AS, OS> Default for DataSchemaSubtype<DS, AS, OS> is not used anymore apparently
• impl<DS, AS, OS> Default for UncheckedObjectSchema<DS, AS, OS> is not tested yet

The rest looks fine.