GeneralDecoding.swift performance regression

[tayloraswift]

benchmarks CI caught a major performance regression with commit 59d34c5f8c337b068d07ad5b0e4caa2fd0b8176e

before:

 [15/16] Linking benchmark
Build complete! (50.69s)
swift-json: decoded 38295 messages
foundation: decoded 38295 messages
swift-json decoding time: 0.497709959 seconds
foundation decoding time: 4.90599839 seconds

after:

[15/16] Linking benchmark
Build complete! (45.21s)
swift-json: decoded 38295 messages
foundation: decoded 38295 messages
swift-json decoding time: 3.21054002 seconds
foundation decoding time: 4.539217732 seconds

[tayloraswift]

link #33