Pest grammar/parser

[alensiljak]

Part of #29

FYI, I've created a repo to try out Ledger grammar with pest: https://github.com/ledger-rs/ledger-pest-parser

The idea is to create a CLI app that uses this as a parser and run it on the Ledger suite of tests once the CLI Test Executor is complete.
It can also be used to compare to other implementations both in terms of readability/maintainability and performance.

I'd like to additionally try LALRPOP and perhaps others. There is ANTLR implementation Tackler. The existing ledger parser is using nom. This provides a decent breadth of options to compare.

[alensiljak]

Pest already has a new version (v3) syntax. It has been in preparation since 2018. So, there is intention of moving forward (not 100% compatible with the previous version) but not much progress.
This poses a question of whether a grammar in any specific version of a parser is really maintainable in the long-term and what are the actual benefits vs a custom parser.