Add readable constructors via proc-macro.

[chaoticlonghair]

Before this PR, if u want to construct a fixed uint / hash in compile time (not runtime), u have to write code like:

const X: U128 = U128([0x64, 0]);
let y = U256([0x6bc7_5e2d_6310_0000, 0x5, 0, 0]);
// In fact, rustfmt will put each `0` in a separate line, more ugly than this example
let z = U4096([
    0x00ab_cdef, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0,
]);

const H: H256 = H256([
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff,
    0xff, 0xff, 0xff, 0xff,
]);

But after this PR, u can write code like these examples:

const X: U128 = u128!("100");
let y = u256!("100_000_000_000_000_000_000");
let z = u4096!("0xabcdef");

const H: H4096 = h4096!("0x_ffff_ffff_ffff_ffff");

The function from_bin_str, from_oct_str, from_hex_str, from_dec_str, hash::from_str, hash::from_hex_str, hash::from_trimmed_hex_str and other similar methods will construct variables in runtime. That will be slow and unsafe (panic in compile time is better than throw exceptions in runtime).

But if use there macros, the variables will be constructed in compile time.

Close #4.

[u2]

😢

[chaoticlonghair]

That is why I push this PR. 😎.