configurable stack and heap memory size (#288)

- Allow configuration of the maximum heap and stack size via CLI flags
and JSON input settings.
- Increase the default value for the stack size to 32kb.

---------

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

crates/llvm-context/src/memory.rs

@@ -0,0 +1,21 @@
+//! The compile time PolkaVM memory configuration settings.
+
+use serde::{Deserialize, Serialize};
+
+/// The PolkaVM memory configuration.
+#[derive(Clone, Copy, Debug, Deserialize, Serialize)]
+pub struct MemoryConfig {
+    /// The emulated EVM linear heap memory size in bytes.
+    pub heap_size: u32,
+    /// The PVM stack size in bytes.
+    pub stack_size: u32,
+}
+
+impl Default for MemoryConfig {
+    fn default() -> Self {
+        Self {
+            heap_size: 64 * 1024,
+            stack_size: 32 * 1024,
+        }
+    }
+}