Use ptr::write_bytes instead of mem::zeroed in allocator

[haileys]

Tests currently fail on master on latest Rust with this error:

failures:

---- tree_bitmap::tests::remove stdout ----
thread 'tree_bitmap::tests::remove' panicked at 'attempted to zero-initialize type `&str`, which is invalid', /home/charlie/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:625:9

---- tree_bitmap::tests::len stdout ----
thread 'tree_bitmap::tests::len' panicked at 'attempted to zero-initialize type `&str`, which is invalid', /home/charlie/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:625:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This pull request replaces ptr::write and mem::zeroed with ptr::write_bytes to avoid creating a real object (or reference to such) for which zeroed bytes are invalid.