`base_convert`: Same trick as with `to_base_le`, find the largest power of base

*On 2022-05-28 @recmo wrote in [`b0b5d54`](https://github.com/recmo/uint/commit/b0b5d5401643d19747653934e838604cd59bcd81) “Mod names”:*

Same trick as with `to_base_le`, find the largest power of base
that fits `u64` and accumulate there first.

```rust
base: u64,
digits: I,
) -> Result {
// OPT: Special handling of bases that divide 2^64, and bases that are
// powers of 2.
// OPT: Same trick as with `to_base_le`, find the largest power of base
// that fits `u64` and accumulate there first.
if base < 2 {
return Err(BaseConvertError::InvalidBase(base));
}
let mut result = Self::ZERO;
for digit in digits {

```
*From [`src/base_convert.rs:82`](https://github.com/recmo/uint/blob/b0b5d5401643d19747653934e838604cd59bcd81/src/base_convert.rs#L82)*