refactor(3p/nix/libutil): Replace string2Int & trim functions

Replaces these functions with corresponding functions from Abseil,
namely absl::StripAsciiWhitespace and absl::SimpleAtoi.

In the course of doing this some minor things I encountered along the
way were also refactored.

This also changes the signatures of the various custom readFile
functions to use absl::string_view types.

third_party/nix/src/libexpr/names.cc

@@ -2,6 +2,8 @@
 
 #include <memory>
 
+#include <absl/strings/numbers.h>
+
 #include "util.hh"
 
 namespace nix {
@@ -68,8 +70,8 @@ std::string nextComponent(std::string::const_iterator& p,
 static bool componentsLT(const std::string& c1, const std::string& c2) {
   int n1;
   int n2;
-  bool c1Num = string2Int(c1, n1);
-  bool c2Num = string2Int(c2, n2);
+  bool c1Num = absl::SimpleAtoi(c1, &n1);
+  bool c2Num = absl::SimpleAtoi(c2, &n2);
 
   if (c1Num && c2Num) {
     return n1 < n2;