packfile: improve Index memory representation to be more compact

[dsymonds]

Instead of using a map for offset indexing, use a sorted slice.
Binary searching is fast, and a slice is much more compact.
This has a negligible hit on speed, but has a significant impact on
memory usage, especially for larger repos.

benchmark                         old ns/op     new ns/op     delta
BenchmarkIndexConstruction-12     15506506      14056098      -9.35%

benchmark                         old allocs     new allocs     delta
BenchmarkIndexConstruction-12     60764          60385          -0.62%

benchmark                         old bytes     new bytes     delta
BenchmarkIndexConstruction-12     4318145       3913169       -9.38%

Signed-off-by: David Symonds [email protected]

[mcuadros]

@erizocosmico