Set the default PolkaVM stack and heap size values to 128KB. (#441)

Fixes https://github.com/paritytech/contract-issues/issues/149

A few test cases from v2-periphery-polkadot are hitting the PVM stack
limit when the contracts are compiled using resolc version 0.5.0. The
same tests pass if the contracts are compiled with a resolc version that
includes commit:
https://github.com/paritytech/revive/commit/878ca91689b99220f7ffd40df760613b7314de86.

After a brief discussion with @xermicus, we propose to set the default
PolkaVM stack and heap size values to 128KB, since we have already
encountered tests that hit the stack size limit and, without real world
data, it is difficult to determine the optimal values.

crates/solc-json-interface/src/standard_json/input/settings/polkavm/memory.rs

@@ -2,11 +2,12 @@
 
 use serde::{Deserialize, Serialize};
 
-pub const DEFAULT_HEAP_SIZE: u32 = 64 * 1024;
-pub const DEFAULT_STACK_SIZE: u32 = 32 * 1024;
+pub const DEFAULT_HEAP_SIZE: u32 = 128 * 1024;
+pub const DEFAULT_STACK_SIZE: u32 = 128 * 1024;
 
 /// The PolkaVM memory configuration.
 #[derive(Clone, Copy, Debug, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
 pub struct MemoryConfig {
     /// The emulated EVM linear heap memory size in bytes.
     #[serde(skip_serializing_if = "Option::is_none")]

crates/solc-json-interface/src/standard_json/input/settings/polkavm/mod.rs

@@ -9,6 +9,7 @@ pub mod memory;
 
 /// PVM specific compiler settings.
 #[derive(Clone, Copy, Default, Debug, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
 pub struct PolkaVM {
     /// The PolkaVM target machine memory configuration settings.
     #[serde(skip_serializing_if = "Option::is_none")]