solc-json-interface: do not unconditionally skip serialization of custom keys (#337)

The data structure can be used to build the JSON input for `resolc` too.
In that case serializing of provided custom options should not be
dismissed.

Makes the memory settings struct more modular as a drive-by.

---------

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

crates/llvm-context/src/polkavm/context/function/runtime/entry.rs

@@ -1,6 +1,7 @@
 //! The entry function.
 
 use inkwell::types::BasicType;
+use revive_solc_json_interface::PolkaVMDefaultHeapMemorySize;
 
 use crate::polkavm::context::address_space::AddressSpace;
 use crate::polkavm::context::function::runtime;
@@ -38,9 +39,12 @@ impl Entry {
             context.xlen_type().const_zero(),
         );
 
-        let heap_memory_type = context
-            .byte_type()
-            .array_type(context.memory_config.heap_size);
+        let heap_memory_type = context.byte_type().array_type(
+            context
+                .memory_config
+                .heap_size
+                .unwrap_or(PolkaVMDefaultHeapMemorySize),
+        );
         context.set_global(
             crate::polkavm::GLOBAL_HEAP_MEMORY,
             heap_memory_type,