core: add support for recording 128-bit integers (#2166)

## Motivation

I've received a request at work to record 128-bit integers and realized
that we should probably support recording them.

## Solution

Added two methods to the `Visit` trait, `record_i128` and `record_u128`.
However, I didn't add the size conversions present for 64-bit integers,
as 128-bit integers are, at this time, more specialized.

tracing-core/src/field.rs

@@ -16,9 +16,9 @@
 //! will contain any fields attached to each event.
 //!
 //! `tracing` represents values as either one of a set of Rust primitives
-//! (`i64`, `u64`, `f64`, `bool`, and `&str`) or using a `fmt::Display` or
-//! `fmt::Debug` implementation. `Subscriber`s are provided these primitive
-//! value types as `dyn Value` trait objects.
+//! (`i64`, `u64`, `f64`, `i128`, `u128`, `bool`, and `&str`) or using a
+//! `fmt::Display` or `fmt::Debug` implementation. `Subscriber`s are provided
+//! these primitive value types as `dyn Value` trait objects.
 //!
 //! These trait objects can be formatted using `fmt::Debug`, but may also be
 //! recorded as typed data by calling the [`Value::record`] method on these
@@ -278,6 +278,16 @@ pub trait Visit {
         self.record_debug(field, &value)
     }
 
+    /// Visit a signed 128-bit integer value.
+    fn record_i128(&mut self, field: &Field, value: i128) {
+        self.record_debug(field, &value)
+    }
+
+    /// Visit an unsigned 128-bit integer value.
+    fn record_u128(&mut self, field: &Field, value: u128) {
+        self.record_debug(field, &value)
+    }
+
     /// Visit a boolean value.
     fn record_bool(&mut self, field: &Field, value: bool) {
         self.record_debug(field, &value)
@@ -490,6 +500,8 @@ impl_values! {
     record_u64(usize, u32, u16, u8 as u64),
     record_i64(i64),
     record_i64(isize, i32, i16, i8 as i64),
+    record_u128(u128),
+    record_i128(i128),
     record_bool(bool),
     record_f64(f64, f32 as f64)
 }