Remove mandatory dependency on hashbrown

[matklad]

Currently borsh unconditionally pulls in hashbrown:

borsh-rs/borsh/Cargo.toml

Line 25 in 8ff7f18

hashbrown = "0.9.1"

That's ungreat:

• hash brown is a big dependency, so it imposes compilation time tax on consumers
• hash brown is a public dependency, so it makes it harder to keep our own backwards compat promise
• the particular version of hashbrown we use is quite old

cc #46

[frol]

Resolved with #155