cmd/compile: improve efficiency of binary export position encoding

cmd/compile: improve efficiency of binary export position encoding

Use -64 instead of 0 as the magic "new file"
line delta, since it is much less common.

Use a new path encoding that breaks up paths
into /-separated components, allowing
reuse of the component strings, and making
many re-used paths a single byte to encode.

Bump the export version to 5.

Fixes #20080

name        old export-bytes  new export-bytes  delta
Template          19.1k ± 0%        17.4k ± 0%  -8.74%  (p=0.008 n=5+5)
Unicode           4.47k ± 0%        4.42k ± 0%  -0.96%  (p=0.008 n=5+5)
GoTypes           29.9k ± 0%        27.6k ± 0%  -7.41%  (p=0.008 n=5+5)
Compiler          71.4k ± 0%        65.4k ± 0%  -8.45%  (p=0.008 n=5+5)
SSA               67.8k ± 0%        65.6k ± 0%  -3.38%  (p=0.008 n=5+5)
Flate             4.99k ± 0%        4.79k ± 0%  -3.91%  (p=0.008 n=5+5)
GoParser          8.77k ± 0%        7.97k ± 0%  -9.14%  (p=0.008 n=5+5)
Reflect           6.27k ± 0%        6.13k ± 0%  -2.22%  (p=0.008 n=5+5)
Tar               9.46k ± 0%        8.82k ± 0%  -6.69%  (p=0.008 n=5+5)
XML               16.0k ± 0%        14.9k ± 0%  -6.69%  (p=0.008 n=5+5)
[Geo mean]        14.8k             14.0k       -5.80%

Change-Id: Iea0c8c62e61dbab3cfd14ee121e34845c85f00d2
Reviewed-on: https://go-review.googlesource.com/41619
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>