chore(sails): display slice as hex w/o alloc

[vobradovich]

Resolves #938 .

Custom Display impl w/o alloc String

Before & after change

[ping_pong_low_level_works] Gas:     1184375865, Size: 115278

[ping_pong_low_level_works] Gas:     1179374017, Size: 114348

[techraed]

I'd remove the printlns and changes in gtest tests, as, imo, recorded profits after benchmark seems to be enough and there's no reason to adjust the tests for that.

[techraed]

Looking at #954 I came up to a new idea: name all these tests with "bench" suffix, so when finishing #934 they will be handled (moved or something else) accordingly