implement memory.fill and memory.copy

[zeldovich]

This adds support for memory.copy and memory.fill opcodes that are used by recent WASM toolchains, including clang.

[jaybosamiya]

Thanks Nickolai! Similar to #5 (comment) it'd be helpful to know the version so I can do a quick test before merging.

I think the code in this PR generally seems reasonable but OOB traps are a concern that I have for both of the operations (ref. https://webassembly.github.io/spec/core/exec/instructions.html#xref-syntax-instructions-syntax-instr-memory-mathsf-memory-copy-x-1-x-2) that might need to be fixed before merging.

[zeldovich]

Here's a small test case that demonstrates what this PR enables, on Arch Linux:

% cat example.c
void _start(void) {
  char buf[256];
  __builtin_memset(buf, 0, sizeof(buf));
}
% clang --version
clang version 21.1.8
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
% clang --target=wasm32 -nostdlib -o example.wasm example.c && wasm-strip example.wasm
%

this uses wasm-strip to get around the names section issue fixed by PR #5.

On the current latest rWasm, I get:

% cargo run example.wasm
...
Finished reading
Error:
   0: Invalid saturation trunctation 0xb

Location:
   src/parser.rs:563

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
% 

with this PR's change, I get:

% cargo run example.wasm
...
Finished reading
Finished parsing
Finished generating
Finished reformatting
Finished
% 

[zeldovich]

Regarding OOB traps, I was expecting this would be handled by Vec's index_mut and copy_within trapping, but maybe there's something more subtle going on with how rWasm expect to trap OOB operations.