perf: optional fast_hash feature for a faster (non-DoS-resistant) hasher

[dsherret]

Bench	Default (SipHash)	fast_hash (FxHash)	Speedup
package_json_value	17,306 ns	10,856 ns	−37.3% (1.59×)
key_heavy_json_value	7,937,820 ns	5,728,275 ns	−27.8% (1.39×)
citm_catalog_json_large_value	24,256,990 ns	20,389,000 ns	−15.9% (1.19×)
tsconfig_json_value	2,176 ns	2,119 ns	−2.6%

This was done before in #16 , but not as a feature flag, which I prefer for this.