Add an API for lambda expressions [blocks: #4651]

[tautschnig]

This is factored out from/in preparation of #4651, which will eventually also add testing for this code.

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 548e8e2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/112369892
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[antlechner]

@tautschnig This change is causing problems in TG, and it took me a while to understand it because I was thinking about lambdas in general (= any kind of anonymous function) and didn't understand why the documentation and constructor were about array types only. @smowton explained to me that these "lambda_exprts" aren't general lambdas, but they are used for the specific case of defining an array using a kind of list comprehension. I think it would be clearer to name them array_comprehension_exprt, even if their ID_lambda and related functions can't be changed for compatibility reasons. I'll make a PR for the renaming.