Add helper methods to symbols

[mjhouse]

Add various helper methods to SymbolItem that will allow users to quickly check types, binding, get the body of functions, get data from object symbols etc. Note: In the following list, &binary and &mut binary mean the overall ELF binary struct that will be written as part of issue #24.

• [x] Add data/data_mut methods
  • [x] Returns the body of function-type symbols
  • [x] Returns the data of object-type symbols (as strings/integers etc?)
• [x] Add section/section_mut methods
  • Get the section of the symbol given a &binary/&mut binary param
• [ ] ~Add name and set_name methods to get the string name~
  • ~Rename current name method to be name_index or something~
  • ~The name and set_name methods take a &binary/&mut binary~
• [x] Add type and binding check methods
  • [x] is_function (true if STT_FUNC)
  • [x] is_object (true if STT_OBJECT)
  • [x] is_section (true if STT_SECTION)
  • [x] is_file (true if STT_FILE)
  • [x] is_common (true if STT_COMMON)
  • [x] is_tls (true if STT_TLS)
  • [x] is_global (true if STB_GLOBAL)
  • [x] is_weak (true if STB_WEAK)
  • [x] is_local (true if STB_LOCAL)

[mjhouse]

Resolved by #52