Add githooks

Add `githooks` directory and:

- Copy the default pre-commit hook into new githooks directory
- Add a call to clippy to the pre-commit hook
- Add a section to the README instructing devs how to use the new
  githooks

README.md

@@ -22,6 +22,17 @@ Contributions to this library are welcome. A few guidelines:
 * No crypto should be implemented in Rust, with the possible exception of hash functions. Cryptographic contributions should be directed upstream to libsecp256k1.
 * This library should always compile with any combination of features on **Rust 1.41.1**.
 
+### Githooks
+
+To assist devs in catching errors _before_ running CI we provide some githooks. If you do not
+already have locally configured githooks you can use the ones in this repository by running, in the
+root directory of the repository:
+```
+git config --local core.hooksPath githooks/
+```
+
+Alternatively add symlinks in your `.git/hooks` directory to any of the githooks we provide.
+
 ## Fuzzing
 
 If you want to fuzz this library, or any library which depends on it, you will

githooks/pre-commit

@@ -0,0 +1,50 @@
+#!/bin/sh
+#
+# A hook script to verify what is about to be committed.
+# Called by "git commit" with no arguments.  The hook should
+# exit with non-zero status after issuing an appropriate message if
+# it wants to stop the commit.
+
+if git rev-parse --verify HEAD >/dev/null 2>&1
+then
+	against=HEAD
+else
+	# Initial commit: diff against an empty tree object
+	against=$(git hash-object -t tree /dev/null)
+fi
+
+# If you want to allow non-ASCII filenames set this variable to true.
+allownonascii=$(git config --bool hooks.allownonascii)
+
+# Redirect output to stderr.
+exec 1>&2
+
+# Cross platform projects tend to avoid non-ASCII filenames; prevent
+# them from being added to the repository. We exploit the fact that the
+# printable range starts at the space character and ends with tilde.
+if [ "$allownonascii" != "true" ] &&
+	# Note that the use of brackets around a tr range is ok here, (it's
+	# even required, for portability to Solaris 10's /usr/bin/tr), since
+	# the square bracket bytes happen to fall in the designated range.
+	test $(git diff --cached --name-only --diff-filter=A -z $against |
+	  LC_ALL=C tr -d '[ -~]\0' | wc -c) != 0
+then
+	cat <<\EOF
+Error: Attempt to add a non-ASCII file name.
+
+This can cause problems if you want to work with people on other platforms.
+
+To be portable it is advisable to rename the file.
+
+If you know what you are doing you can disable this check using:
+
+  git config hooks.allownonascii true
+EOF
+	exit 1
+fi
+
+# If there are whitespace errors, print the offending file names and fail.
+git diff-index --check --cached $against -- || exit 1
+
+# Check that code lints cleanly.
+cargo clippy --features=rand-std,recovery,lowmemory,global-context --all-targets -- -D warnings || exit 1