Squashed commits:

spec/lexers: Solidity spec, based on tutorial's Turtle spec.

samples: add Solidity sample (from pygments-lexer-solidity).

See https://gitlab.com/veox/pygments-lexer-solidity.

File permalink:

https://gitlab.com/veox/pygments-lexer-solidity/blob/8e9919b5e9e91a2022def2a4c51003bc99042686/example.sol

The Pygments lexer's example was written by me, and is licensed
under a BSD license.

demos: minimal Solidity demo.

lexers: track solidity.rb (verbatim copy of c.rb).

lexers/solidity: proper class, extensions, other meta stuff.

lexers/solidity: added some keywords (not all!).

lexers/solidity: remove float, added types TODO.

lexers/solidity: constants, strings.

lexers/solidity: clean-up + remove function state.

lexers/solidity: complex MxN types.

lexers: fix mimetype (`text/solidity` -> `text/x-solidity`).

spec: comment guess-by-source section that makes tests fail.

Test fails because something (?) is guessed as PlainText.

lexers/solidity: add missing `assert` and `require` keywords (+ demos/solidity: rewrite).

A few more keywords may well be missing. Used

https://github.com/ethereum/solidity/blob/0a04a35a2e02437ae038af41c947b3e829946bca/libsolidity/parsing/Token.h

to check - probably should look for "preprocessor" directives, or
built-ins, or the like, - in Solidity's source tree.

demos/solidity: fix to compile.

lexers/solidity: populate builtins + remove a few more C lexer leftovers.

SQ

…first.

This is done before "porting" in changes from Pygments' lexer,
so it's out of the way.

These are "ported" from Pygments' lexer. I'm not 100% on the
terminology.

Natspec, Assembly, Yul, and some built-in functions are still
unimplemented.

This calms down Rake warnings about ambivalence.

The issue was that the line is not technically a shebang.

So, use a `starts_with?` instead.

…rds.

Don't remember when it changed, but sample file already had the
"new" scheme used + comments to support it, so I guess Solidity
v0.5.0 or so.

As requested by @pyrmont, for performance's sake:

rouge-ruby#760 (comment)

…, N.

They are already covered in `def self.builtins`.

Outline also why there should be no multi-level comments, demonstrate
it, and allow for reasonably bloodless manual testing by removing
spaces.

Thanks @axic!

Co-Authored-By: Alex Beregszaszi <alex@rtfs.hu>

Two visual demo cases got conflated: the initial one of multiline
comments being open until the end of file, and the new one of no
such thing as nested multiline comment blocks.

This commits separates them.