[Variant] Minor: use From impl to make conversion infallable (apache#8068)

# Which issue does this PR close?

- Follow on to  apache#8044

# Rationale for this change

@scovich had a good suggestion here
https://github.com/apache/arrow-rs/pull/8044/files#r2257256848:
> value.into() makes clear that the conversion is infallible?


# What changes are included in this PR?

Use `From` impl to make it clear the conversion is infallible and can
not lose precision

# Are these changes tested?

Covered by existing tests

# Are there any user-facing changes?

No

Author: alamb

parquet-variant/src/variant.rs

@@ -1155,7 +1155,7 @@ impl From<u8> for Variant<'_, '_> {
         if let Ok(value) = i8::try_from(value) {
             Variant::Int8(value)
         } else {
-            Variant::Int16(value as i16)
+            Variant::Int16(i16::from(value))
         }
     }
 }
@@ -1166,7 +1166,7 @@ impl From<u16> for Variant<'_, '_> {
         if let Ok(value) = i16::try_from(value) {
             Variant::Int16(value)
         } else {
-            Variant::Int32(value as i32)
+            Variant::Int32(i32::from(value))
         }
     }
 }
@@ -1176,7 +1176,7 @@ impl From<u32> for Variant<'_, '_> {
         if let Ok(value) = i32::try_from(value) {
             Variant::Int32(value)
         } else {
-            Variant::Int64(value as i64)
+            Variant::Int64(i64::from(value))
         }
     }
 }
@@ -1188,7 +1188,7 @@ impl From<u64> for Variant<'_, '_> {
             Variant::Int64(value)
         } else {
             // u64 max is 18446744073709551615, which fits in i128
-            Variant::Decimal16(VariantDecimal16::try_new(value as i128, 0).unwrap())
+            Variant::Decimal16(VariantDecimal16::try_new(i128::from(value), 0).unwrap())
         }
     }
 }