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/attr-path.cc

@@ -1,5 +1,7 @@
 #include "attr-path.hh"
 
+#include <absl/strings/numbers.h>
+
 #include "eval-inline.hh"
 #include "util.hh"
 
@@ -50,7 +52,7 @@ Value* findAlongAttrPath(EvalState& state, const std::string& attrPath,
     /* Is i an index (integer) or a normal attribute name? */
     enum { apAttr, apIndex } apType = apAttr;
     unsigned int attrIndex;
-    if (string2Int(attr, attrIndex)) {
+    if (absl::SimpleAtoi(attr, &attrIndex)) {
       apType = apIndex;
     }
 

third_party/nix/src/libexpr/get-drvs.cc

@@ -4,6 +4,7 @@
 #include <regex>
 #include <utility>
 
+#include <absl/strings/numbers.h>
 #include <glog/logging.h>
 
 #include "derivations.hh"
@@ -243,7 +244,7 @@ NixInt DrvInfo::queryMetaInt(const std::string& name, NixInt def) {
     /* Backwards compatibility with before we had support for
        integer meta fields. */
     NixInt n;
-    if (string2Int(v->string.s, n)) {
+    if (absl::SimpleAtoi(v->string.s, &n)) {
       return n;
     }
   }

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;