closes TravonteD#9

An escaped backspace just before the closing quote would be recognized
as an escaped quote and the rest of your program becomes a giant string.

Fennel on its own doesn't really deal with escape sequences, but the
underlying Lua platform does.  We can't really know its version so we
just support the latest, 5.4.

Numbers can start or end on just a dot (.003, 100.)

Fixing just that would be boring, so here's some new features:
* hexadecimal literals: 0xdeadbeef
* exponents, decimal and hexadecimal: 1e10, 0x1p10
* underscores as numerical separators: 1_000.000_03

The first two are Lua features.  Hexadecimal integer literals were
introduced in Lua 5.1 and hexadecimal float literals in Lua 5.2.  There
have been no further additions up to Lua 5.4.

The last feature is all Fennel and it's pretty insane.  For example,
here are some ways to represent the number 32:
* 0x20
* 0x10p1
* +_0_x_1_0_p_+_1

The last one doesn't even look like a number.  For my own sanity, I
only allow underscores between digits and not directly after the dot,
the exponent "character" (the 'e', 'E', 'p', 'P') or any sign.

Not really broken but there's definitely more characters we can allow.
In fact, we allow everything!  With some exceptions.

I only noticied this by chance: `(foo. bar . baz)` would get parsed as a
single `field_expression`, because of how `seq()`s work.

We can use the same technique as with `field_expression`s to correctly
parse methods and only methods.  We need to set a higher precedence on
the colon specifically to "overpower" `field`

There is no reason for it to be a separate rule.

Any quoted special form loses its special meaning, recursively.

Our first conflicts!  The alternative would be to add precendece in all
the binding rules and I don't really want to do that.

If they just take statements they're not really worth their own rule.

";" would get parsed as a string with only its starting quote and then a
comment.  The ending quote would not be found and all the rest of your
program is a string.

I though `token.immediate` was supposed to not allow...

It is still supported and used in the Fennel compiler

They're just symbols.

However, they are symbols with dots in them (at least `.` and `..` are).
Yet, they're not multisyms either.  Indeed, a symbol that begins or ends
on a dot is not a multisym.

I managed to get the starting dots working.  Unfortunately, I can't get
a symbol to only end on a dot without completely breaking
`multi_symbol`.  So, no `?.` for you. :(