[performance] Redundant source-map encoding roundtrip

[mourner]

Noticed this huge low hanging fruit while profiling source maps:

src/node-entry.ts → dist/rollup.js, dist/rollup.es.js...

magic-string: chunk: 65.457ms
magic-string: encode: 144.542ms
rollup: decode: 27.857ms
rollup: collapse: 109.229ms

magic-string: chunk: 41.675ms
magic-string: encode: 36.950ms
rollup: decode: 38.783ms
rollup: collapse: 255.896ms

created dist/rollup.js, dist/rollup.es.js in 4.3s

src/browser-entry.ts → dist/rollup.browser.js...

magic-string: chunk: 50.308ms
magic-string: encode: 216.066ms
rollup: decode: 32.689ms
rollup: collapse: 68.621ms

rollup: created dist/rollup.browser.js in 2.6s

When magic-string generates source maps for Rollup, it encodes the mappings array into a spec-compliant string, but then rollup immediately decodes it for further manipulation (collapsing stage). If we introduce a non-encoded version of MagicString generateMap, we can eliminate the steps in bold above entirely, cutting the source map step almost in half.

Stand by while I'm working on corresponding PRs.

[mourner]

Upstream PR here Rich-Harris/magic-string#134
Once released, will submit a Rollup PR (already have it working locally).

[marvinhagemeister]

@mourner Nothing to add, just want you to know that I love your recent PRs 🍀 Thanks for making rollup faster for all of us 👍