llvm-context: lazy handling of function arguments and immutable data (#282)

- Lazily load function arguments so that they can be passed as pointers.
- Lazily call the immutable store function to avoid storing zero sized
immutable data.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/integration/codesize.json

@@ -1,10 +1,10 @@
 {
-  "Baseline": 1443,
-  "Computation": 2788,
-  "DivisionArithmetics": 9748,
-  "ERC20": 19150,
-  "Events": 2201,
-  "FibonacciIterative": 2041,
-  "Flipper": 2691,
-  "SHA1": 8997
+  "Baseline": 1365,
+  "Computation": 2710,
+  "DivisionArithmetics": 9672,
+  "ERC20": 19131,
+  "Events": 2123,
+  "FibonacciIterative": 1964,
+  "Flipper": 2590,
+  "SHA1": 8918
 }