tidy up the runtime API crate (#85)

- remove unused runtime API imports and constants
- move runtime api symbols into the revive-runtime-api crate

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

crates/integration/codesize.json

@@ -1,10 +1,10 @@
 {
-  "Baseline": 983,
-  "Computation": 4207,
-  "DivisionArithmetics": 40509,
-  "ERC20": 47068,
-  "Events": 1791,
-  "FibonacciIterative": 3044,
-  "Flipper": 3405,
-  "SHA1": 33583
+  "Baseline": 981,
+  "Computation": 4145,
+  "DivisionArithmetics": 40606,
+  "ERC20": 47335,
+  "Events": 1773,
+  "FibonacciIterative": 3027,
+  "Flipper": 3385,
+  "SHA1": 33545
 }