bump version to 0.15.5

Revert "Add a rand-std feature"

845bff25 Bump version to 0.15.4 (Steven Roose)
07ef3a1f Pin version of build dependency cc to <1.0.42 (Steven Roose)

Pull request description:

  Version 1.0.42 broke compatibility with rustc 1.22.0.

ACKs for top commit:
  jonasnick:
    ACK 845bff25

Tree-SHA512: 21161e6e85494d064b45b9800baa0355784e3b47772dff2716f049b1b15c047832ceb756b2031ef73374952c21aa04a8852a333eb3edbb73f5608c7ce8344c12

Version 1.0.42 broke compatibility with rustc 1.22.0.

Add a rand-std feature

Explicit checks for ZST + null fallbacks

Remove TODOs for secp256k1 callbacks

We would not want to use these functions internally because we rely on
USE_EXTERNAL_DEFAULT_CALLBACKS to provide the callbacks at link time,
see f7a4a7ef. Moreover, we would not
want to export the functions either.

Remove all usage of `mem::uninitialized()`

Signed-off-by: Elichai Turkel <[email protected]>

Signed-off-by: Elichai Turkel <[email protected]>

Optimizing travis build time and fixing benchmarks

Cache cargo-web only

build.rs: Add feature 'lowmemory' to reduce memory usage

And fill in release date for 0.15.2.

Currently, this only set `ECMULT_WINDOW_SIZE` to 4 instead of 15.

Fixes #139.

fixup

Fix typos

Propagating std to rand dependency

bump version to 0.15.0

Relax cc requirements more

 Implement hex human-readable serde for PublicKey 

Add links manifest key to prevent cross-version linkage