Directly read blob contents without copying to memory

[at-wat]

Reduce peak memory usage during map load.

Previous implementation copied whole JS blob contents to memory and read via bytes.Reader.
New implementation adds a wrapper to directly read JS blob contents as io.Reader.

So, the peak memory usage during map load should be halved.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.16%. Comparing base (fb95330) to head (c59d5e2).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   38.16%   38.16%           
=======================================
  Files           8        8           
  Lines        1643     1643           
=======================================
  Hits          627      627           
  Misses        980      980           
  Partials       36       36           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[f-fl0]

LGTM