test-suite feature: generate well-typed Exprs

[mhuesch]

this is a little tricky.
two approaches come to mind:

1. generate terms using the untyped generator, call infer them on them, & discard not-well-typed ones.

• seems inefficient - we will probably rarely generate a well-typed Expr of any non-miniscule size.

2. define a new type-directed generator.

• seems hard, but doable.
• here we might generate a Type first, and then use that to guide generation of an Expr which fits the type.
• I imagine there's research literature about doing this, but I am currently ignorant of it.

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migrated from mhuesch/poly-rs#13.