Rewrite `zap_srcref()` in C

[lionel-]

Closes #1513.

@wch could you try this branch out and see if it fixes your performance issues please?

[wch]

This does indeed speed up the startup time for shiny app I was testing before.

A simple benchmark without shiny:

library(rlang)
system.time({
  for (i in 1:1000) {
    zap_srcref(lm)
  }
})

Results with CRAN version:

   user  system elapsed 
  0.891   0.015   0.910 

Results with this PR:

   user  system elapsed 
  0.017   0.001   0.017