Refactor BootstrapMethodInvoker to further avoid runtime type checks

We can improve startup efficiency of the BootstrapMethodInvoker by adding back explicit casts to MethodType or Class as appropriate, and improve the structure of how we special case known BSMs.

Adding a special case for StringConcatFactory BSMs while we're at it can reduce bootstrap overhead of using indify string concat by a sizeable chunk (~200k executed bytecode).

PoC patch: http://cr.openjdk.java.net/~redestad/scratch/scf_bsm.patch