Reduce build time of `TextTable.renderTableHeader()`
Reduce build time of `TextTable.renderTableHeader()`

On MacBook Pro (Retina, 13-inch, Late 2013) 2.6 GHz Intel Core i5,
cumulative time is reduced from 2186.8ms to 29.0ms. (by Build Time Analyzer)

scottrhoyt/SwiftyTextTable,scottrhoyt/SwiftyTextTable,scottrhoyt/SwiftyTextTable