allow dynamic configuration of the heap memory (#287)

This PR changes the implementation of the emulated EVM heap memory:
Instead of linking in a C implementation the code is emitted directly
into the contract module. Which allows making it configurable via a
compiler parameter (a follow up PR to this).

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

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

@@ -5,6 +5,7 @@ pub mod deploy_code;
 pub mod entry;
 pub mod revive;
 pub mod runtime_code;
+pub mod sbrk;
 
 /// The main entry function name.
 pub const FUNCTION_ENTRY: &str = "__entry";