feat: replace special-cased block call method with primitive and remove source length parameter

Remove the `sourceLength` field from the parser and `compile()` signature, switching to null-terminated strings read by `readFile()`. Replace the special `METHOD_CALL` method type with a `block_call` primitive that invokes `callBlock()` on the fiber, and move the `Fiber` and `CallFrame` structs into the public header so primitives can access them. Add a test for block calling via the new primitive.

test/block_call.wren

@@ -0,0 +1 @@
+io.write({ 123 }.call) // expect: 123