test: add fuzzers and support for fuzzing testing

There is a number of bugs related to parsing and encoding/decoding data.
Examples:

- csv: #2692, #4497, #2692
- uri: #585

One of the effective method to find such issues is a fuzzing testing.
Patch introduces a CMake flag to enable building fuzzers (ENABLE_FUZZER)
and add fuzzers based on LibFuzzer [1] to csv, http_parser and uri
modules. Note that fuzzers must return 0 exit code only, other exit
codes are not supported [2].

NOTE: LibFuzzer requires Clang compiler.

1. https://llvm.org/docs/LibFuzzer.html
2. http://llvm.org/docs/LibFuzzer.html#id22

How-To Use:

$ mkdir build && cd build
$ cmake -DENABLE_FUZZER=ON \
	-DENABLE_ASAN=ON \
	-DCMAKE_BUILD_TYPE=Debug \
	-DCMAKE_C_COMPILER="/usr/bin/clang" \
	-DCMAKE_CXX_COMPILER="/usr/bin/clang++" ..
$ make -j
$ ./test/fuzz/csv_fuzzer -workers=4 ../test/static/corpus/csv

Part of #1809