Don't enable bitvec when "std" feature is enabled

[jsdw]

Currently we have:

std = [
    "bitvec/std",
    "scale/std",
]

As the std feature in Cargo.toml. Enabling it enables bitvec too.

Let's swap to:

std = [
    "bitvec?/std",
    "scale/std",
]

To avoid enabling bitvec unless it's also asked for?