Improve prior_trap processing

We use `trap` to bootstrap our installation function (__bp_install). We
remove our code upon first execution but need to restore any preexisting
trap calls. We previously used `sed` to process the trap string, but
that had two downsides:

1. `sed` is an external command dependency. It needs to exist on the
   system, and we need to invoke it in a subshell (which has some
   runtime cost).
2. The regular expression pattern was imperfect and didn't handle
   trickier cases like `'` characters in the trap string:

        $ (trap "echo 'hello'" DEBUG; trap -p DEBUG)
        hello
        trap -- 'echo '\''hello'\''' DEBUG

This change removes the dependency on `sed` by locally evaluating the
trap string and extracting any prior trap. This works reliably because
we control the format our trap string, which looks like this (with
newlines expanded):

    __bp_trap_string="$(trap -p DEBUG)"
    trap - DEBUG
    __bp_install

Author: jparise

bash-preexec.sh

@@ -297,10 +297,8 @@ __bp_install() {
     trap '__bp_preexec_invoke_exec "$_"' DEBUG
 
     # Preserve any prior DEBUG trap as a preexec function
-    local prior_trap
-    # we can't easily do this with variable expansion. Leaving as sed command.
-    # shellcheck disable=SC2001
-    prior_trap=$(sed "s/[^']*'\(.*\)'[^']*/\1/" <<<"${__bp_trap_string:-}")
+    eval "local trap_argv=(${__bp_trap_string:-})"
+    local prior_trap=${trap_argv[2]:-}
     unset __bp_trap_string
     if [[ -n "$prior_trap" ]]; then
         eval '__bp_original_debug_trap() {