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_set.zig

@@ -0,0 +1,61 @@
+const HashMap = @import("hash_map.zig").HashMap;
+const mem = @import("mem.zig");
+const Allocator = mem.Allocator;
+
+pub const BufSet = struct {
+    hash_map: BufSetHashMap,
+
+    const BufSetHashMap = HashMap([]const u8, void, mem.hash_slice_u8, mem.eql_slice_u8);
+
+    pub fn init(allocator: &Allocator) -> BufSet {
+        var self = BufSet {
+            .hash_map = undefined,
+        };
+        self.hash_map.init(allocator);
+        return self;
+    }
+
+    pub fn deinit(self: &BufSet) {
+        var it = self.hash_map.entryIterator();
+        while (true) {
+            const entry = it.next() ?? break; 
+            self.free(entry.key);
+        }
+
+        self.hash_map.deinit();
+    }
+
+    pub fn put(self: &BufSet, key: []const u8) -> %void {
+        if (self.hash_map.get(key) == null) {
+            const key_copy = %return self.copy(key);
+            %defer self.free(key_copy);
+            %return self.hash_map.put(key_copy, {});
+        }
+    }
+
+    pub fn delete(self: &BufSet, key: []const u8) {
+        const entry = self.hash_map.remove(key) ?? return;
+        self.free(entry.key);
+    }
+
+    pub fn count(self: &const BufSet) -> usize {
+        return self.hash_map.size;
+    }
+
+    pub fn iterator(self: &const BufSet) -> BufSetHashMap.Iterator {
+        return self.hash_map.entryIterator();
+    }
+
+    fn free(self: &BufSet, 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: &BufSet, value: []const u8) -> %[]const u8 {
+        const result = %return self.hash_map.allocator.alloc(u8, value.len);
+        mem.copy(u8, result, value);
+        return result;
+    }
+};
+