Tests for 6.3.4 Characters

Test:138     Success:84     Failure:0     Error:54
write methods for characters

fix a bug in test-eval.rb
Test:138     Success:90     Failure:0     Error:48
add functions for 6.3.4 Characters

char->integer, integer->char
Test:138     Success:92     Failure:0     Error:46
Functions for 6.3.4 Characters

Add functions specified in section 6.3.4(Characters)
Fix bugs in test-eval.rb (function names are wrong)

Test:142     Success:133     Failure:0     Error:9