Enable composing components that use GC

[gertvv]

For example, plugging a component with wasm-gc internals into a wasi-cli command:

$ wac plug command-incomplete.wasm --plug gc_component.wasm  -o command.wasm
error: struct indexed types not supported without the gc feature (at offset 0x16)

I couldn't find any flag to enable GC support.

[tanishiking]

@gertvv It is fixed by #146
Unfortunately, the new version hasn't yet released, use cargo install --git https://github.com/bytecodealliance/wac wac-cli

[calvinrp]

Let me know if this works in the latest 0.7.0 release.