libexpr: fix various overflows and type mismatches (backport #13309)

[mergify]

Motivation

I found wrong casts of NixInt values, which can cause crashes of current Nix versions because of sizeof(unsigned int) < sizeof(NixInt::Inner), e.g.

nix-repl> builtins.substring 4294967296 5 "hello"  
error: basic_string_view::substr: __pos (which is 4294967296) > __size (which is 5)

Furthermore usually sizeof(unsigned int) < sizeof(size_t) on 64 bit systems, which becomes problematic when a list contains more than 2^32 values and this case isn't impossible. Thus size_t is used as index type.

Context

Includes tests for cases, where NixInt::Inner was incorrectly casted.

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This is an automatic backport of pull request #13309 done by [Mergify](https://mergify.com).

[mergify]

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On branch mergify/bp/2.29-maintenance/pr-13309
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	both modified:   src/libexpr-tests/primops.cc
	both modified:   src/libexpr/primops.cc

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