zig build system improvements, add some std API

* add std.buf_map.BufMap
 * add std.buf_set.BufSet
 * add std.mem.split
 * zig build system improvements (See #204)
   - automatically parses NIX_CFLAGS_COMPILE and NIX_LDFLAGS
   - add builder.addCIncludePath
   - add builder.addRPath
   - add builder.addLibPath
   - add exe.linkLibrary

std/buf_map.zig

@@ -0,0 +1,71 @@
+const HashMap = @import("hash_map.zig").HashMap;
+const mem = @import("mem.zig");
+const Allocator = mem.Allocator;
+
+/// BufMap copies keys and values before they go into the map, and
+/// frees them when they get removed.
+pub const BufMap = struct {
+    hash_map: BufMapHashMap,
+
+    const BufMapHashMap = HashMap([]const u8, []const u8, mem.hash_slice_u8, mem.eql_slice_u8);
+
+    pub fn init(allocator: &Allocator) -> BufMap {
+        var self = BufMap {
+            .hash_map = undefined,
+        };
+        self.hash_map.init(allocator);
+        return self;
+    }
+
+    pub fn deinit(self: &BufMap) {
+        var it = self.hash_map.entryIterator();
+        while (true) {
+            const entry = it.next() ?? break; 
+            self.free(entry.key);
+            self.free(entry.value);
+        }
+
+        self.hash_map.deinit();
+    }
+
+    pub fn set(self: &BufMap, key: []const u8, value: []const u8) -> %void {
+        if (const entry ?= self.hash_map.get(key)) {
+            const value_copy = %return self.copy(value);
+            %defer self.free(value_copy);
+            %return self.hash_map.put(key, value_copy);
+            self.free(entry.value);
+        } else {
+            const key_copy = %return self.copy(key);
+            %defer self.free(key_copy);
+            const value_copy = %return self.copy(value);
+            %defer self.free(value_copy);
+            %return self.hash_map.put(key_copy, value_copy);
+        }
+    }
+
+    pub fn delete(self: &BufMap, key: []const u8) {
+        const entry = self.hash_map.remove(key) ?? return;
+        self.free(entry.key);
+        self.free(entry.value);
+    }
+
+    pub fn count(self: &const BufMap) -> usize {
+        return self.hash_map.size;
+    }
+
+    pub fn iterator(self: &const BufMap) -> BufMapHashMap.Iterator {
+        return self.hash_map.entryIterator();
+    }
+
+    fn free(self: &BufMap, value: []const u8) {
+        // remove the const
+        const mut_value = @ptrcast(&u8, value.ptr)[0...value.len];
+        self.hash_map.allocator.free(mut_value);
+    }
+
+    fn copy(self: &BufMap, value: []const u8) -> %[]const u8 {
+        const result = %return self.hash_map.allocator.alloc(u8, value.len);
+        mem.copy(u8, result, value);
+        return result;
+    }
+};