make dynamic vars private and add new fns and macros for interacting with them:

Fns for retrieving the current value of the dyn vars
* inactive-node-modification-error
* inactive-buffer-modification-error
* block-node-until-ready

Macros for binding the den vars to specific values for a specific body
* with-inactive-node-modification-error
* with-inactive-buffer-modification-error
* with-inactive-modification-error
* without-node-blocking