Update num-bigint requirement from 0.3.1 to 0.4.0 in /boa

[dependabot]

Updates the requirements on num-bigint to permit the latest version.

Changelog

Sourced from num-bigint's changelog.

Release 0.4.0 (2021-03-05)

Breaking Changes

• Updated public dependences on arbitrary, quickcheck, and rand:
  • arbitrary support has been updated to 1.0, requiring Rust 1.40.
  • quickcheck support has been updated to 1.0, requiring Rust 1.46.
  • rand support has been updated to 0.8, requiring Rust 1.36.
• Debug now shows plain numeric values for BigInt and BigUint, rather than the raw list of internal digits.

Contributors: @​cuviper, @​Gelbpunkt

Release 0.3.2 (2021-03-04)

• The new BigUint methods count_ones and trailing_ones return the number of 1 bits in the entire value or just its least-significant tail, respectively.
• The new BigInt and BigUint methods bit and set_bit will read and write individual bits of the value. For negative BigInt, bits are determined as if they were in the two's complement representation.
• The from_radix_le and from_radix_be methods now accept empty buffers to represent zero.
• BigInt and BigUint can now iterate digits as u32 or u64, regardless of the actual internal digit size.

Contributors: @​BartMassey, @​cuviper, @​janmarthedal, @​sebastianv89, @​Speedy37

Release 0.3.1 (2020-11-03)

• [Addition and subtraction now uses intrinsics][141] for performance on x86 and x86_64 when built with Rust 1.33 or later.
• [Conversions to_f32 and to_f64 now return infinity][163] for very large numbers, rather than None. This does preserve the sign too, so a large negative BigInt will convert to negative infinity.
• [The optional arbitrary feature implements arbitrary::Arbitrary][166], distinct from quickcheck::Arbitrary.
• [The division algorithm has been optimized][170] to reduce the number of temporary allocations and improve the internal guesses at each step.
• [BigInt and BigUint will opportunistically shrink capacity][171] if the internal vector is much larger than needed.

... (truncated)

Commits

• d1e4498 Merge #197
• 4b7c9d7 Fix clippy::needless_range_loop
• a49c0ac Release 0.4.0
• 7c40fde Merge #194 #195
• 96cf255 Update to arbitrary-1 and quickcheck-1
• 60637c5 Merge #185
• 4247abe implement xorshift* for benchmarking
• 998d4cb seed tests with u64 for platform consistency
• 46aa212 Bump to 0.4.0-pre
• 63a8967 Update for rand's MSRV 1.36
• Additional commits viewable in compare view

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[dependabot]

Looks like num-bigint is no longer updatable, so this is no longer needed.