[Rust] Fix function/method discovery from harness

[arthurscchan]

This PR fixes the logic and adds more visit implementations to enable the Rust frontend to discover nested or chained function/method calls within the fuzz_target macro of the fuzzing harness.

[DavidKorczynski]

please add sample improvements in the text. Such as from the httparse project

[arthurscchan]

Originally, some nested and chained functions/methods were missing from the generated call tree. For example, when parsing this fuzzing harness in the httparse project, the call graph only contained the following result because chained function/method calls were ignored:

Call tree
fuzz_target /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=-1
  Response<'h,'b>::new /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=7

This PR updates the handling and discovery of function/method calls within nested or chained structures, ensuring all functions/methods are detected. The updated call tree now shows:

Call tree
fuzz_target /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=-1
  ParserConfig::allow_multiple_spaces_in_response_status_delimiters /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=9
  Response<'h,'b>::new /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=7
  ParserConfig::parse_response /src/httparse/fuzz/fuzz_targets/parse_response_multspaces.rs linenumber=10