fix(toDecimal): preserve negative sign for leading zeros

[jparise]

Change #690 fixed the general case of formatting negative unit values, but was one remaining case where formatting was still incorrect: when the first unit is -0, the resulting string didn't include the leading negative sign.

This change identifies that exact case (negative value, leading zero) and pads the resulting string with a leading negative sign.

Fixes #692

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[jparise]

Given that getDecimal always receives a two-element units array, I took a more "drastic" approach and unrolled the .map() operation. We were specializing each element (first, last) within the mapping function anyway.

(getDecimal could assert(units.length === 2, ...), but I think the assert() in toDecimal already covers this "must be decimal" case?)

I think this results in a simpler solution overall that only uses a single top-level branch (to determine if we need to add the leading negative).

[johnhooks]

@jparise it reads very clearly to me.

[sarahdayan]

Excellent rewrite! A few suggestions and we'll be good to go.

[sarahdayan]

Awesome contribution, thanks @jparise!