add expression transformers
[bugfix] wrong conditions to enable expression transformation
classify basic block objects
explicitly return from L_exit basic block even if the return value is of void
transformer is now capable of choosing the strategy of goto elimination
no need to return from end points of case clause
[bugfix] fix a crash on break/continue statements with a label
add another strategy to eliminate gotos
Merge branch 'wasabiz/experiments/expression-cps' into wasabiz/experiments/expression-cps2

Conflicts:
	src/instruments.jsx
fix bugs introduced by the previous merge
add transformed function elements getters
use performCPSTransformation
remove YieldStatement. add YieldExpression instead
give identity function as expression transformer root
unfold CPS expressions
remove debug print wrongly introduced by the last commit
change Generator class definition to fit with yield expression
convert yield expression
add yield expression test
Merge branch 'master' into wasabiz/experiments/yield-expression