Small optimization in Parquet varint decoder (#8742)

# Which issue does this PR close?

- Part of #5853.

# Rationale for this change

Following the recent improvements in Thrift decoding, the percentage of
time spent decoding LEB128 encoded integers has increased.

# What changes are included in this PR?

This PR modifies the varint decoder to first test for integers that can
be encoded in a single byte (using zig-zag encoding, the maximum int
that can be encoded is 63). Many of the fields in the Parquet footer
(including all enum values) will be in this range, so optimizing for
this frequent occurrence makes sense.

# Are these changes tested?

Should be covered by existing tests

# Are there any user-facing changes?

No

Author: etseidl

parquet/src/parquet_thrift.rs

@@ -276,8 +276,13 @@ pub(crate) trait ThriftCompactInputProtocol<'a> {
 
     /// Read a ULEB128 encoded unsigned varint from the input.
     fn read_vlq(&mut self) -> ThriftProtocolResult<u64> {
-        let mut in_progress = 0;
-        let mut shift = 0;
+        // try the happy path first
+        let byte = self.read_byte()?;
+        if byte & 0x80 == 0 {
+            return Ok(byte as u64);
+        }
+        let mut in_progress = (byte & 0x7f) as u64;
+        let mut shift = 7;
         loop {
             let byte = self.read_byte()?;
             in_progress |= ((byte & 0x7F) as u64).wrapping_shl(shift);