Replace cast `as *const i8` with `as *const c_char`.
Replace cast `as *const i8` with `as *const c_char`.

This solves the following type mismatch error in 32-bit ARM systems.

error[E0308]: mismatched types
key.as_ptr() as *const i8,
^^^^^^^^^^^^^^^^^^^^^^^^^ expected u8, found i8

kaedroho/rust-rocksdb,tempbottle/rust-rocksdb,spacejam/rust-rocksdb,tempbottle/rust-rocksdb,spacejam/rust-rocksdb,spacejam/rust-rocksdb,kaedroho/rust-rocksdb,kaedroho/rust-rocksdb,tempbottle/rust-rocksdb