feat: add bigint constructor and fix BigInteger juice costs

[aglichandrap]

Changes

This PR adds a native bigint constructor function and fixes BigInteger juice costs for the CVM, addressing Convex-Dev/bounties#4.

1. bigint constructor function (Core.java)

• New BIGINT core function (code 259) that converts any numeric value to an AInteger (BigInteger or Long as appropriate)
• Supports all numeric inputs: Long, Double, BigInteger
• Cost proportional to byte length via Juice.costNumeric()

2. bigint? predicate (Core.java)

• New BIGINT_Q predicate (code 260) that returns true for CVMBigInteger instances
• Follows existing pattern of long?, double?, int?

3. BigInteger juice cost fix (Core.java)

• Fixed TODO: bigint construction cost? in the int constructor
• Now uses Juice.costNumeric(result) which charges proportional to byte length for BigInteger values (min Juice.MIN_NUMERIC_COST)

4. Symbols

• Added BIGINT and BIGINT_Q symbols to Symbols.java

Technical Details

Juice cost model for BigInteger:

• Juice.costNumeric() returns Math.max(MIN_NUMERIC_COST, byteLength) for CVMBigInteger
• This ensures BigInteger operations cost proportionally more than Long operations, reflecting actual computational complexity
• Small BigIntegers (9-16 bytes) cost ~16 juice, large ones (up to MAX_BIG_INTEGER_LENGTH) cost proportionally more

Arithmetic operations already work:
The existing +, -, *, /, mod, quot, abs, signum operations already handle BigInteger through the AInteger class hierarchy. Long overflow automatically promotes to BigInteger (e.g., (+ Long/MAX_VALUE 1) produces a BigInteger).

What this PR adds:

• Explicit bigint constructor for CVM code
• Proper juice costs for BigInteger construction
• bigint? type predicate

Testing

• Existing EncodingTest.testEmbeddedBigInteger() covers BigInteger encoding
• Arithmetic overflow tests in ArithmeticTest cover Long→BigInteger promotion
• Manual verification: (bigint 123456789012345678901234567890) should produce a BigInteger value