Adding usage examples

Add tests for no-std

Reintroduce rand-std feature

Add a feature to disable replacing C symbols with rust

Add a 'dont_replace_c_symbols' feature for not replacing C's symbols with rust. isn't compatible with no-std

Disable some useless lints

Remove always debug mode

This is checked automatically using the DEBUG env variable

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()`