Auto merge of rust-lang#136974 - m-ou-se:fmt-options-64-bit, r=scottmcm

Reduce FormattingOptions to 64 bits

This is part of rust-lang#99012

This reduces FormattingOptions from 6-7 machine words (384 bits on 64-bit platforms, 224 bits on 32-bit platforms) to just 64 bits (a single register on 64-bit platforms).

Before:

```rust
pub struct FormattingOptions {
    flags: u32, // only 6 bits used
    fill: char,
    align: Option<Alignment>,
    width: Option<usize>,
    precision: Option<usize>,
}
```

After:

```rust
pub struct FormattingOptions {
    /// Bits:
    ///  - 0-20: fill character (21 bits, a full `char`)
    ///  - 21: `+` flag
    ///  - 22: `-` flag
    ///  - 23: `#` flag
    ///  - 24: `0` flag
    ///  - 25: `x?` flag
    ///  - 26: `X?` flag
    ///  - 27: Width flag (if set, the width field below is used)
    ///  - 28: Precision flag (if set, the precision field below is used)
    ///  - 29-30: Alignment (0: Left, 1: Right, 2: Center, 3: Unknown)
    ///  - 31: Always set to 1
    flags: u32,
    /// Width if width flag above is set. Otherwise, always 0.
    width: u16,
    /// Precision if precision flag above is set. Otherwise, always 0.
    precision: u16,
}
```

Author: bors

core/src/fmt/float.rs

@@ -86,7 +86,7 @@ where
         true => flt2dec::Sign::MinusPlus,
     };
 
-    if let Some(precision) = fmt.options.precision {
+    if let Some(precision) = fmt.options.get_precision() {
         float_to_decimal_common_exact(fmt, num, sign, precision)
     } else {
         let min_precision = 0;
@@ -162,7 +162,7 @@ where
         true => flt2dec::Sign::MinusPlus,
     };
 
-    if let Some(precision) = fmt.options.precision {
+    if let Some(precision) = fmt.options.get_precision() {
         // 1 integral digit + `precision` fractional digits = `precision + 1` total digits
         float_to_exponential_common_exact(fmt, num, sign, precision + 1, upper)
     } else {
@@ -180,7 +180,7 @@ where
         true => flt2dec::Sign::MinusPlus,
     };
 
-    if let Some(precision) = fmt.options.precision {
+    if let Some(precision) = fmt.options.get_precision() {
         // this behavior of {:.PREC?} predates exponential formatting for {:?}
         float_to_decimal_common_exact(fmt, num, sign, precision)
     } else {