fix(compiler): convert Big* values by value into native int/float/bool slots

[Giandonn]

Fixes #137

A BigInt, BigFloat or Decimal value is a php::Box resource. When one reached a native int, float or bool slot, convertExprType() called convertFloatExpr() / convertIntExpr() / convertBoolExpr() without the source type, so the generic php::toFloat() / php::toInt() ran zval_get_double() / zval_get_long() on the resource and produced its handle. Since a float literal with 16+ fractional digits is recognized as a Decimal literal, ordinary PHP code hits this:

function takesFloat(float $x): float { return $x; }
takesFloat(0.123456789012345678);
// PHP:      float(0.12345678901234568)
// compiled: float(4)   <- the box resource handle

The change

• convertExprType() forwards a Big* source type, so the existing per-type conversion (php::Decimal::toFloat() etc.) is used for arguments, typed returns and fixed native locals.
• Those conversions return a php::Variant, which does not convert to a native C++ scalar, so the Big* branch of convertFloatExpr() / convertIntExpr() / convertBoolExpr() now unwraps it with php::toFloat() / php::toInt() / php::toBool(). Before, that branch only produced valid C++ where a Variant was accepted, which is why nothing exercised it for native slots.

Explicit ->toFloat() calls and Big* arithmetic are untouched.

Verified on a binary

PHP 8.4.26 ZTS with embed, GCC 11, Linux x64. New PHPT tests/compiler/bignumber/native-scalar-conversions.phpt (arguments of all three Big* types into float/int/bool, typed returns, a fixed native local):

	master	this PR
native-scalar-conversions.phpt	FAIL: float(4), float(5), float(6), float(7), int(8), int(9), …	PASS
tests/compiler/bignumber + tests/compiler/operator	—	59/59 PASS
bigint, decimal, std-bigint, std-bigfloat, std-decimal, var_convert, type_decl, float_edge, universal_method, optimizations, function, functions	—	169 PASS, 1 skipped (also skipped on master)
PHPUnit BigInt|BigFloat|Decimal|BigNumber|Conversion|Cast|TypedScalar|NativeScalar|NativeType	—	58 tests OK

Not covered

A Decimal literal handed to a dynamic Zend context still stays a box (sprintf('%.5f', 0.123456789012345678) prints 10.00000; an array element holds resource(…) of type (php::box)). That is a separate decision about how auto-detected Decimal literals should behave outside native slots, so it is left out of this PR and noted in #137.