use 'lgrow' helper for growing langsxps

[kevinushey]

Fixes #1386.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[eddelbuettel]

Nice work. And echos nicely my general sentiment on pairlists I read somewhere (WRE?) about being )(and I paraphrase) 'rarely used and for internals only' 😜

[eddelbuettel]

Clean reverse-depends check with just one issue also appearing at CRAN (i.e. 'not new'): RcppCore/rcpp-logs@2fd6614