Java: [BC] Implement type assertion table

[Bryce McKinlay]

For the BC-ABI, it isn't sufficient to determine type compatibility at 
compile type - if the class heirarchy changes at runtime such that a 
given class is no longer assignment compatible with another class, type 
safety could be violated. Instead, the runtime must be able to detect 
incompatible changes and throw a verification error during class 
initialization.

This patch implements type assertions for the new verifier, and changes 
the way type assertions are implemented. Previously, where we'd emit an 
artificial method containing the assertions as code, we now generate a 
table of simple assertion entries, consisting of a 'type code' and up to 
two string operands. This gives the runtime additional flexibility in 
how the assertions are evaluated, as well saving space in the binary and 
making it easier to ensure backwards-compatibility for binaries.

OK to commit?

Bryce

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