Implement methods of String

[seven1m]

Our main approach is to work on one spec from ruby/spec at a time, implementing as much of the spec as possible, then check it off the list:

• allocate_spec.rb
• append_spec.rb
• ascii_only_spec.rb
• b_spec.rb
• byteindex_spec.rb
• byterindex_spec.rb
• bytesize_spec.rb
• byteslice_spec.rb
• bytesplice_spec.rb
• bytes_spec.rb
• capitalize_spec.rb
• casecmp_spec.rb
• case_compare_spec.rb
• center_spec.rb
• chars_spec.rb
• chilled_string_spec.rb
• chomp_spec.rb
• chop_spec.rb
• chr_spec.rb
• clear_spec.rb
• clone_spec.rb
• codepoints_spec.rb
• comparison_spec.rb
• concat_spec.rb
• count_spec.rb
• crypt_spec.rb
• dedup_spec.rb
• delete_prefix_spec.rb
• delete_spec.rb
• delete_suffix_spec.rb
• downcase_spec.rb
• dump_spec.rb
• dup_spec.rb
• each_byte_spec.rb
• each_char_spec.rb
• each_codepoint_spec.rb
• each_grapheme_cluster_spec.rb
• each_line_spec.rb
• element_reference_spec.rb
• element_set_spec.rb
• empty_spec.rb
• encode_spec.rb
• encoding_spec.rb
• end_with_spec.rb
• eql_spec.rb
• equal_value_spec.rb
• force_encoding_spec.rb
• freeze_spec.rb
• getbyte_spec.rb
• grapheme_clusters_spec.rb
• gsub_spec.rb
• hash_spec.rb
• hex_spec.rb
• include_spec.rb
• index_spec.rb
• initialize_spec.rb
• insert_spec.rb
• inspect_spec.rb
• intern_spec.rb
• length_spec.rb
• lines_spec.rb
• ljust_spec.rb
• lstrip_spec.rb
• match_spec.rb
• modulo_spec.rb
• multiply_spec.rb
• new_spec.rb
• next_spec.rb
• oct_spec.rb
• ord_spec.rb
• partition_spec.rb
• plus_spec.rb
• prepend_spec.rb
• replace_spec.rb
• reverse_spec.rb
• rindex_spec.rb
• rjust_spec.rb
• rpartition_spec.rb
• rstrip_spec.rb
• scan_spec.rb
• scrub_spec.rb
• setbyte_spec.rb
• size_spec.rb
• slice_spec.rb
• split_spec.rb
• squeeze_spec.rb
• start_with_spec.rb
• string_spec.rb
• strip_spec.rb
• sub_spec.rb
• succ_spec.rb
• sum_spec.rb
• swapcase_spec.rb
• to_c_spec.rb
• to_f_spec.rb
• to_i_spec.rb
• to_r_spec.rb
• to_s_spec.rb
• to_str_spec.rb
• to_sym_spec.rb
• tr_spec.rb
• tr_s_spec.rb
• try_convert_spec.rb
• uminus_spec.rb
• undump_spec.rb
• unicode_normalized_spec.rb
• unicode_normalize_spec.rb
• unpack1_spec.rb
• unpack_spec.rb
• upcase_spec.rb
• uplus_spec.rb
• upto_spec.rb
• valid_encoding_spec.rb

Note: there is another issue for String#unpack here: #667

[ryangjchandler]

Both String#lstrip and String#rstrip are spec-compliant!

[ryangjchandler]

String#size is also spec-compliant (minus the encoding tests).

[ryangjchandler]

String#center is spec-compliant since #518 too :)