Measuring things

The first of these next commands was in LaTeX~2.09. The two new commands are the obvious analogues.

174 {<<#339#>length-cmd<#339#>>} {<<#340#>lr text<#340#>>}
175 {<<#341#>length-cmd<#341#>>} {<<#342#>lr text<#342#>>}
176 {<<#343#>length-cmd<#343#>>} {<<#344#>lr text<#344#>>}