#24 Implement bytecode verifier

[Sami Koivu]

A bytecode verifier should be implemented which would
warn about problems in classes (but which would allow
creating/saving/opening/modifying invalid classes anyway).

Stuff to warn about:

*Invalid max stack change values (requires
implementation of
http://sourceforge.net/tracker/index.php?func=detail&aid=1518171&group_id=122082&atid=692314\)

*Invalid references (index or type) in constant pool

*Missing returns in code blocks

[Sami Koivu]

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Continuing the list:
*Invalid pc values in the LineNumberTable

[Sami Koivu]

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More information:

See chapter "4.9 Format Checking" and 4.10 and 4.11 of the
Java Classfile format documentation for Java 1.5