Use lamdas in helper functions

[priyasiddharth]

context: f0a72b3#r44268019

I tried various things

1. use boost::hana anonymous/lambda functions
2. use c++14 lambda functions

The generated bitcode seems correct but is not correctly processed by our pipeline. Some issues I see are

1. Some hana idioms generate a lot of code e.g. while_, but some don't e.g. times_with_index
2. in some cases the generated functions in bitcode don't follow C naming and hence are not found by BMC engine
3. even if they are found, they have some trouble accessing values in the creation context. Specifically

[&](auto i) {
  if (str[i] == '\0' and !found) {
    sassert(0);  \\ never fires
    length = i
    found = true
  }
}

Figuring why 3 was happening seemed out of scope to me so I went with a basic solution that did not rely on these features.

from @agurfinkel
ok. something to revisit during the break. Remind me. I can dig into bitcode to look what is going on.
SeaHorn works on C++ code. The C naming convention should not be related.