bv_arithmetict: Lift precondition to from_expr's interface

[tautschnig]

Only constant_exprt are accepted, thus input parameter's type should guarantee
this. Also update to_expr, which can only generate constant_exprt. Note that
neither of these functions is currently used in the code base.

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[kroening]

More spaces needed for clang-format!

[tautschnig]

Not sure why this is happening, but sometimes the check appears to be confused about the branches to check, this is not a change in this PR. Will rebase.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 4051607).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/93290725