Implement IoURI with parser and formatter

[gavv]

IoURI is an URI for devices and files. See #258.

Changes:

• add ragel support to scons
• add roc_address module
• add IoURI
• add parse_io_uri() (ragel-based) and format_io_uri()
• add io_uri_to_str
• add pct_encode() and pct_decode() and use them in IoURI

The integration (roc_sndio, tools, documentation) will be done in next PRs.

See also:

• https://en.wikipedia.org/wiki/File_URI_scheme
• https://tools.ietf.org/html/rfc8089
• https://tools.ietf.org/html/rfc3986

[gavv]

The build will fail until I'll add ragel to docker images.