std.fmt.parseHexFloat: clean up bitwise logic

 * fold a couple separate operations into one
 * use const instead of var
 * naming conventions

Author: andrewrk

lib/std/fmt/parse_hex_float.zig

@@ -206,13 +206,11 @@ pub fn parseHexFloat(comptime T: type, s: []const u8) !T {
     //   - we've truncated more than 0.5ULP (R=S=1)
     //   - we've truncated exactly 0.5ULP (R=1 S=0)
     // Were are going to increase the mantissa (round up)
-    var exactly_half = (mantissa & 0b11) == 0b10;
-    var more_than_half = (mantissa & 0b11) == 0b11;
+    const guard_bit_and_half_or_more = (mantissa & 0b110) == 0b110;
     mantissa >>= 2;
-    var guardBit = mantissa & 1 == 1;
     exponent += 2;
 
-    if (guardBit and (exactly_half or more_than_half)) {
+    if (guard_bit_and_half_or_more) {
         mantissa += 1;
     }