Stack trace domain explosion

[jcheng5]

This is part of the fix for https://github.com/jcheng5/shinychat/issues/16

• Investigate putting a cap on deep stack traces, at least by default Limit deep stack growth #4156
• Add unit test

Simple repro (watch the dots get slower and slower):

library(shiny)
library(promises)
library(coro)

ui <- fluidPage(
  
)

server <- function(input, output, session) {
  observe({
    coro::async(function () {
      for (i in 1:1e3) {
        await(coro::async_sleep(0))
        cat(".")
        if (i %% 50 == 0) cat("\n")
      }
    })()
  })
}

shinyApp(ui, server)

[jcheng5]

@cpsievert will review post-merge