Use tinyvec's rustc_1_55 feature to reduce compilation time

While casually reading
https://hackmd.io/mxdn4U58Su-UQXwzOHpHag (Analysis of
rustc-benchmarking-data), I noticed that it mentions
Lokathor/tinyvec#161.

The tinyvec crate, with the default set of features, can compile on
rustc < 1.55 by using large implementations of Default for [T; N].  On
Rustc >= 1.55, it can elide this code and just use const generics.
Apparently this reduces compilation time for tinyvec by about half -
which is not a lot in the context of librsvg, but let's make use of
that.

Librsvg currently requires Rust >= 1.64.0 anyway, so we can make use
of tinyvec's rustc_1_55 feature.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/822>

Author: federicomenaquintero

rsvg/Cargo.toml

@@ -75,7 +75,7 @@ regex = "1.7.1"
 rgb = { version="0.8", features=["argb"] }
 selectors = "0.24.0"
 string_cache = "0.8.0"
-tinyvec = { version = "1.2.0", features = ["alloc"] }
+tinyvec = { version = "1.2.0", features = ["alloc", "rustc_1_55"] }
 url = "2"
 xml5ever = "0.17.0"
 yeslogic-fontconfig-sys = { version = "4.0.1", optional = true }