Add support for lambdas with implicit boxing

Turns out the usual lambda metafactory can apply boxing/unboxing, so you might see a situation like

int myMethod(int i) { ... }

interface MyLambdaType { Integer f(Integer i); }

MyLambdaType mlt = SomeType::myMethod;

And contrary to my prior expectations, the compiler doesn't generate a method stub to do the type conversion,
it just directly requests an invokestatic lambda pointed at myMethod, regardless of the int vs. Integer clash.
The lambda metafactory (which re-implement in JBMC) is then responsible for adding the necessary conversions.

Author: smowton

jbmc/regression/jbmc/lambda-boxing/Boolean.desc

@@ -0,0 +1,9 @@
+CORE
+BoxingLambda
+--function BoxingLambda.testBoolean --cp `../../../../scripts/format_classpath.sh . ../../../lib/java-models-library/target/core-models.jar`
+\[java::BoxingLambda\.testBoolean:\(\)V\.assertion\.1\] line 73 assertion at file BoxingLambda\.java line 73 function java::BoxingLambda\.testBoolean:\(\)V bytecode-index 25: SUCCESS
+\[java::BoxingLambda\.testBoolean:\(\)V\.assertion\.2\] line 74 assertion at file BoxingLambda\.java line 74 function java::BoxingLambda\.testBoolean:\(\)V bytecode-index 35: SUCCESS
+\[java::BoxingLambda\.testBoolean:\(\)V\.assertion\.3\] line 75 assertion at file BoxingLambda\.java line 75 function java::BoxingLambda\.testBoolean:\(\)V bytecode-index 47: SUCCESS
+\[java::BoxingLambda\.testBoolean:\(\)V\.assertion\.4\] line 76 assertion at file BoxingLambda\.java line 76 function java::BoxingLambda\.testBoolean:\(\)V bytecode-index 53: FAILURE
+^EXIT=10$
+^SIGNAL=0$