BTreeMap: enforce the panic rule imposed by `replace`
BTreeMap: enforce the panic rule imposed by `replace`

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust