Define different types of blocks

[dethe]

Instead of BlockDef we should have

• [x] StepDef
• [x] ValueDef
• [x] ContextDef
• [x] TriggerDef

[dethe]

Step and Value blocks are two views on the same type of block, but we can provide hints for their default view.

[dethe]

For what it's worth, Scratch defines:

• 8 Hat (Trigger) blocks (when flag clicked, when key pressed, when sprite clicked, when I receive msg, when backdrop switched to X, when X > Y, when I start as a clone)
• 5 C (Context) blocks: forever loop, repeat x, repeat until x, if x then, if x then else
• 34 Reporter (Value returning number or string) blocks
• 14 Boolean (Value returning boolean) blocks
• 3 Cap (Steps with no next step) blocks: Stop (x), delete this clone, loop forever.
• 62 Stack (Step) blocks

[dethe]

Implemented, need to add tests for AST before calling it done.