Speed up TSFASTImporter (from 7sec to 115ms for a python file of 900 LoC)

[jecisc]

I tried to import some files with TSFASTImporter without doing any customizations to the FAST model and I got disappointed because it was already taking 7sec for a file of 900LoC and this is slow :(

I am proposing a big speed up in this PR.

Previously, for each node we were using either the TSPoint or the #start/endByte to compute the positions in number of characters. But the more nodes we have, the slower it is getting.

Since all nodes are computing from the same origin string, I propose to compute only once a dictionary associating the index of each leading bytes to the index of the corresponding characters. This takes 4ms for a file a 900 lines (~23k bytes) containing characters encoded on multiple bytes and then getting the positions in characters from the position in bytes becomes almost instant.

The change brings the import of the file from 7sec to 115ms on my machine.

Fixes #32 by making it obsolete

[jecisc]

I updated the comment of the PR because I added a commit that changes a lot the intent behind this PR :)

[jecisc]

I did some benchmark before and after this PR on this file:

https://raw.githubusercontent.com/DefinitelyTyped/DefinitelyTyped/master/types/react/index.d.ts

I had some errors so I added a try/catch in the visit node to not fail.

Before the PR it was taking 124sec and after the PR it takes 400ms :)

I also tried to import this file:

https://github.com/microsoft/TypeScript/blob/main/src/compiler/parser.ts

Before the PR I stopped the import after 5minutes. After the PR it takes 3seconds :)