Move parser to solely use $ for variables for 2021 edition support

[udoprog]

We have to stop using # as a delimiter, because the 2021 edition reserved any identifier-prefixed # tokens making expressions like these no longer be legal:

quote!(hello#world);

This would be seen as the hello#world token by Rust 2021, and since the hello prefix is not recognized by the Rust compiler and would not compile. See https://doc.rust-lang.org/edition-guide/rust-2021/reserving-syntax.html for more.

$ does not suffer from this (and is more in the spirit of macros) so we're now moving all syntax to use it even if that means it's a bit more verbose.

This implements the following parser changes:

• #var is now $var.
• #(var + 5) is now $(var + 5)
• #_(hello world) is now $[str](hello world).
• #_(hello $world) is now $[str](hello $world) (runtime evaluation, like having hello ${world} in javascript).
• #_(hello $(world + 1)) is now $[str](hello $(world + 1)) (runtime evaluation, like having hello ${world + 1} in javascript).
• #_(hello #world) is now $[str](hello $[const](world)) (compile time evaluation, whatever value world has will become a component of the generated string).

The following control sequences are changed:

• #<space> becomes $[' ']
• #<push> becomes $['\r'].
• #<line> becomes $['\n'].

This change will be part of the genco 0.17 release.

[udoprog]

This is being tested in https://github.com/reproto/reproto/tree/genco-2021