Make number of declared structs contextual

[frabert]

Fixes #293

[github-actions[bot]]

See the diff generated by this PR for the tests here: https://github.com/lifting-bits/rellic/actions/runs/2902212007

### fizzbuzz.bc ```diff ``` ### typedefs_of_typedefs.bc ```diff ``` ### switch_loop.bc ```diff ``` ### func_cond_zero_arg.bc ```diff ``` ### fcmp.bc ```diff ``` ### conflicting_names.bc ```diff ``` ### struct.bc ```diff ``` ### issue_183_literal_structs.bc ```diff ``` ### zeroinit.bc ```diff ``` ### issue_127_uint128_t_lit.bc ```diff ``` ### struct_swap.bc ```diff ``` ### init_list.bc ```diff ``` ### ret0.bc ```diff ``` ### bool.bc ```diff ``` ### short.bc ```diff ``` ### cast.bc ```diff ``` ### nullptr.bc ```diff ``` ### nested_struct.bc ```diff ``` ### array_swap.bc ```diff ``` ### binops.bc ```diff ``` ### bitmask.bc ```diff ``` ### loop.bc ```diff ``` ### float.bc ```diff ``` ### reg_test_structure_fields.bc ```diff ``` ### vectors.bc ```diff ``` ### funcptr.bc ```diff ``` ### goto_loop.bc ```diff ``` ### branch.bc ```diff ``` ### bitops.bc ```diff ``` ### trunc.bc ```diff ``` ### inttoptr.bc ```diff ``` ### issue_4.bc ```diff ``` ### issue_123_uint128_t.bc ```diff ``` ### fizzbuzz_stateful.bc ```diff ``` ### issue_94_strncmp.bc ```diff ``` ### global_using_function_decl.bc ```diff ``` ### byval_struct.bc ```diff ``` ### nested_while.bc ```diff ``` ### func_cond_two_arg.bc ```diff ``` ### conflicting_global.bc ```diff ``` ### assert.bc ```diff ``` ### zext.bc ```diff ``` ### switch.bc ```diff ``` ### template_parameter_pack.bc ```diff ``` ### byval_tail_nogep.ll ```diff ``` ### byval_tail_gep.ll ```diff ```