support runtime addition of reader macros

[obriencj]

similar to the stepped way that compilation happens at the module level, where each top-level statement can modify the environment to add new compile-able forms and macros for the next expression

support reader macros such that additional behavior can be grafted on.

move existing reader behavior into reader macros as possible!

[obriencj]

things that can be implemented as reader macros

• cons
• string
• quote
• quasiquote, unquote, unquote splicing
• python list literals [x y z]
• python map literals {x: x y: y z: z}
• python set literals {x y z}
• atom

this should allow for discarding the entire sibilant.ast module, so long as I can ensure position information is stored in a lookaside during parsing.

the end result would be a stream plus a locations dict passed into a reader function which had a collection of reader macros. the reader function would use the macros is had available to parse and return a single expression.

compilation and execution could then happen, at which time more reader macros might be defined. subsequent reader calls would then operate with the new macros in place.

[obriencj]

(set-macro-character "{" handler_fn) where handler_fn accepts parameters as (lambda (stream trigger_char) ...) and returns a pair representing the form that it should evaluate as. The macro can read from stream to advance it as needed.

[obriencj]

This relies on a function named "read" which connects to the reader and pulls in a single expression/unit/whatever

[obriencj]

https://gist.github.com/obriencj/ed947df0d8b843ff0e7f6018ae5d79de