perf: Remove unnecessary allocation and construction of a HashMap in Signature::parse #1076

Job	Run time
format	19s
format	19s
clippy (async-std-surf)	1m 4s
clippy (async-std-surf)	1m 4s
clippy-codegen	28s
clippy-codegen	28s
test (async-std-surf)	3m 34s
test (async-std-surf)	3m 34s
clippy (tokio-hyper)	55s
clippy (tokio-hyper)	55s
verify-codegen	56s
verify-codegen	1m 3s
test (tokio-hyper)	4m 43s
test (tokio-hyper)	4m 7s
clippy (tokio-hyper-rustls)	1m 8s
clippy (tokio-hyper-rustls)	1m 8s
test (tokio-hyper-rustls)	3m 54s
test (tokio-hyper-rustls)	3m 54s
clippy (tokio-hyper-rustls-webpki)	1m 12s
clippy (tokio-hyper-rustls-webpki)	1m 12s
test (tokio-hyper-rustls-webpki)	3m 45s
test (tokio-hyper-rustls-webpki)	3m 45s
clippy (blocking)	48s
clippy (blocking)	48s
test (blocking)	4m 39s
test (blocking)	4m 12s
clippy (blocking-rustls)	1m 11s
clippy (blocking-rustls)	1m 11s
clippy (blocking-rustls-webpki)	1m 6s
clippy (blocking-rustls-webpki)	1m 6s
test (blocking-rustls)	4m 43s
test (blocking-rustls)	4m 19s
test (blocking-rustls-webpki)	3m 34s
test (blocking-rustls-webpki)	3m 34s
	1h 14m 38s

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