Remove `ExactSizeIterator` constraint from `SimpleSmt::with_leaves()` #806

Job	Run time
Build stable on ubuntu with --no-default-features --target wasm32-unknown-unknown	27s
Test stable on ubuntu with --features default,std,serde	48s
Clippy with --features default,std,serde	31s
Build nightly on ubuntu with --no-default-features --target wasm32-unknown-unknown	28s
Test stable on ubuntu with --no-default-features	46s
Clippy with --no-default-features	33s
rustfmt	14s
Test nightly on ubuntu with --features default,std,serde	52s
Test nightly on ubuntu with --no-default-features	51s
	5m 30s

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